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What is surface hardening

What is surface hardening

Surface quenching refers to the heat treatment process that the treated workpiece is heated to above the phase transformation point within the limited depth range of the surface, and then cooled rapidly to achieve the purpose of quenching within a certain depth range of the workpiece surface.
For iron and steel materials, surface quenching refers to the rapid heating of the steel surface above AC3 (hypoeutectoid steel) or AC1 (hypereutectoid steel) with a special heating method, followed by rapid cooling, so that martensitic transformation occurs on the steel surface and a hardened layer is formed.

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The purpose is to improve the hardness, wear resistance and fatigue strength of the workpiece surface, while the core still has high toughness. Commonly used in shaft, gear and other parts. During operation, the surface layer of the workpiece is austenitized by rapid heating, and then quenched immediately to change the surface structure into martensite, and the core structure is basically unchanged. Low temperature tempering is generally carried out after surface quenching. According to different heating methods, it can be divided into induction heating surface quenching, flame heating surface quenching, electrical contact heating surface quenching, electrolyte heating surface quenching, etc., of which the former two methods are most widely used.

Classification of surface quenching

According to different forms of surface energy, it can be divided into:
(1) Induction heating surface quenching
The quenching method of heating the workpiece surface by generating eddy current with high current density on the workpiece surface based on the principle of electromagnetic induction.
(2) Flame quenching
Surface quenching method of directly heating workpiece surface with extremely high temperature combustible gas flame.
(3) Electric contact heating surface quenching
When the electrode with low voltage and high current is introduced into and contacted with the workpiece, the contact resistance between the electrode and the workpiece surface is heated to heat the workpiece surface.
(4) Electrolyte heating surface quenching
A quenching method in which the workpiece is inserted into the electrolyte as an electrode (cathode) and the cathode effect is used to heat the workpiece surface.
(5) Laser heating surface quenching
The focused laser beam is used as the heat source to irradiate on the surface of the workpiece to be treated, so that the temperature of the part to be hardened increases rapidly to form austenite, and then the hardened layer with fine grain martensite or other structures is obtained by rapid cooling.
(6) Electron beam heating surface quenching
When the electron beam bombards the surface in a short time, the surface temperature increases rapidly, while the substrate remains cold. When the electron beam stops bombarding, the heat is rapidly transmitted to the cold base metal, so as to heat the surface and quench itself.
(7) Plasma beam heating surface quenching
The plasma beam with high energy density is used as the heat source to form a supersonic jet, scan the metal surface and make it reach the austenitizing temperature at a very fast speed. The heat source is removed immediately, and the heat is immediately transmitted to the deep part of the workpiece and the unheated part. The local surface of the heated workpiece is cooled rapidly, the austenite in this area is transformed into martensite and strengthened, and the hardness is greatly improved.

Application of surface quenching

It is generally used to treat medium carbon quenched and tempered steel and nodular cast iron.
Common surface hardened steel and cast iron grades

Type

Steel grade

Application

Carbon structural steel

35, 40, 45, 50

Small module, light load gear and shaft parts

Alloy structural steel

40Cr, 45MnB

Medium modulus, light load gear and high strength transmission shaft

30CrMo, 42CrMo,42SiMn

Gears and shafts with large modulus and load

5CrMnMo, 5CrNiMo

Parts with heavy load

Cast iron

Grey cast iron

Machine tool guide rail, cylinder liner

Nodular cast iron, alloy nodular cast iron

Crankshaft, machine tool spindle, camshaft

Principle of surface quenching process

Phase transformation characteristics of steel during nonequilibrium heating

As mentioned above, the basic condition for surface quenching of steel is that there is sufficient energy density to provide surface heating, so that the surface can reach the temperature above the phase transformation point quickly enough. Therefore, during surface quenching, the steel is in non-equilibrium heating.
Steel has the following characteristics during unbalanced heating:
1. In a certain range of heating speed, the critical point increases with the increase of heating speed.
During rapid heating, they move to high temperature with the increase of heating speed. However, when the heating rate reaches a certain range, the transformation temperatures of all hypoeutectoid steels are the same. The faster the heating rate, the wider the temperature range of Austenite Formation, but the faster the formation rate; The formation time is short. The heating speed has little effect on the initial formation temperature of austenite, but with the increase of heating speed, the final formation temperature is significantly increased. The more uneven the original structure is, the higher the final formation temperature is
2. The non-uniformity of austenite composition increases with the increase of heating speed
As mentioned above, with the increase of heating speed, the transformation temperature increases and the transformation temperature range expands. With the increase of transformation temperature, the austenite carbon concentration in equilibrium with ferrite decreases and that in equilibrium with cementite increases. Therefore, the austenite carbon concentration adjacent to ferrite will be very different from that in austenite adjacent to cementite. Due to the fast heating speed, short heating time and too late diffusion of carbon and alloy elements, the composition of austenite will be uneven, and the heterogeneity of austenite will increase with the increase of heating speed. For example, when 0.4% C carbon steel is heated to 900 ℃ at the heating rate of 130 ℃/s, there is a carbon concentration zone of 1.6% C in austenite. Obviously, during rapid heating, the steel grade and original structure have a great influence on the uniformity of austenite composition. It is difficult to use rapid heating for high alloy steel with small heat conductivity, coarse carbide and difficult dissolution
3. Increasing the heating rate can significantly refine the austenite grain
During rapid heating, the superheat is very large, and the austenite nucleation may be formed not only at the ferrite carbide phase interface, but also at the sub grain boundary of ferrite, so the austenite nucleation string increases. Because the heating time is very short, the austenite grain has no time to grow. When ultra fast heating is used, the ultra-fine grain can be obtained.
4. Rapid heating has an obvious effect on the transformation of undercooled austenite and martensite tempering
Rapid heating makes the austenite composition uneven and grain refined, reduces the stability of undercooled austenite and shifts the C curve to the left. Due to the heterogeneity of austenite composition, especially hypoeutectoid steel, two kinds of composition heterogeneity will appear. In the pearlite area, there is compositional heterogeneity between the original cementite area and the original ferrite area, which is very small, that is, the heterogeneity in small volume. However, there is compositional heterogeneity in the original pearlite area and the original eutectoid ferrite block area, which is the heterogeneity in large volume. Due to the large volume heterogeneity of this composition, It will make the martensite transformation points and martensite morphology in these two regions different, that is, it is equivalent to low-carbon martensite in the original ferrite region and high-carbon martensite in the original pearlite region. Due to the non-uniformity of austenite composition after rapid heating, the martensite composition after quenching is also non-uniform. Therefore, although the hardness after quenching is high, the hardness decreases rapidly during tempering, Therefore, the tempering temperature should be slightly lower than that of ordinary heating quenching.

Microstructure and properties of surface quenching

1. Metallographic structure of surface quenching
The metallographic structure of steel parts after surface quenching is related to the steel type, the original structure before quenching and the temperature distribution along the section during quenching and heating. The simplest is eutectoid steel whose original structure is annealed. After quenching, the metallographic structure shall be divided into three zones, which are martensite zone (m) (including retained austenite), martensite plus pearlite (M + P) and pearlite (P) from the surface to the center. Therefore, the martensite pearlite zone appears here. Because austenite is formed in a temperature range and not at a constant temperature during rapid heating, its boundary is equivalent to the initial formation temperature of austenite and the final formation temperature of austenite along the section temperature curve. In the full martensite zone, due to the temperature difference, the difference can also be seen in some cases, The maximum surface temperature is high, the martensite is coarse, the middle is uniform and fine, and the temperature zone is close to the beginning of formation. Due to the uneven austenite composition before quenching, pearlite traces (“pearlite soul”) can be seen if the corrosion is appropriate. When the temperature is lower than the final temperature zone formed by austenite, due to the original annealing structure, no structural change can occur during heating, Therefore, it is the original structure before quenching.

If the original structure of 45# steel is normalized before surface quenching, the change of metallographic structure along the section after surface quenching will be much more complex. If the quenching medium with high quenching intensity is used, that is, as long as the heating temperature is higher than the critical point, all austenitic areas can be quenched into martensite. According to its metallographic structure, it is divided into four areas, surface martensitic area (m), Inside is martensite plus ferrite (M + F), and then to the inside is the martensite plus ferrite plus pearlite zone. The center is equivalent to the temperature lower than that of austenite, and the zone begins to form. The zone is the original structure before quenching, that is, pearlite plus ferrite. In the full martensite zone, the metallographic structure is also significantly different. In the zone close to the transformation point AC3, it is equivalent to the original structure. The ferrite part is a low-carbon martensite zone with deep corrosion color, which is equivalent to the original bead The light martensite zone is a cryptocrystalline martensite zone that is not easy to corrode, and the color depth of the two varies greatly (Fig.1-5b). Thus, it moves to the quenched surface, the low-carbon martensite zone gradually expands and the color gradually becomes lighter, while the color of the cryptocrystalline martensite zone becomes darker and becomes medium carbon martensite near the surface (Fig.1-5a).

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Fig. 1 metallographic structure of different heating temperature zones after surface quenching of steel
If the original structure of 45# steel is quenched and tempered before surface quenching, because the tempered sorbite is a uniform structure with granular carburization uniformly distributed on the ferrite matrix, the non-uniformity of quenched structure caused by the large volume non-uniformity of carbon concentration above will not appear after surface quenching. In the cross section, it is equivalent to ACL and AC3. In the quenched structure in the temperature zone, insoluble sorbite is also divided The cloth is relatively uniform. When the quenching heating temperature is lower than AC1 to the area equivalent to the quenching, tempering and tempering temperature, further tempering will occur because its temperature is higher than the original quenching, tempering and tempering temperature and lower than the critical point. The tempering degree of this part depends on parameter m, and its area size depends on the temperature gradient along the section during surface quenching heating. The faster the heating speed, the steeper the temperature gradient along the section, and the smaller the area. Due to the fast heating speed, short heating time and small parameter m, the tempering degree also decreases.
The depth of surface quenching hardening layer is generally calculated to the semi martensite (50% m) area. The macroscopic measurement method is to prepare metallographic samples along the section and corrode them with nitric acid alcohol. It can be determined according to the color difference between the hardened area and the unhardened area (the color of the hardened area is light); it can also be determined by measuring the section hardness.

Properties after surface quenching

(1) Surface hardness
The surface hardness of the workpiece after rapid heating and quenching is higher than that of ordinary heating quenching. For example, the hardness of 45# steel quenched by laser heating can be 4 Rockwell hardness units higher than that of ordinary quenching; The surface hardness of high frequency heating spray quenching is also 2 ~ 3 Rockwell hardness units higher than that of ordinary heating quenching. This phenomenon of increasing hardness is related to the heating temperature and heating speed. When the heating speed is constant, the phenomenon of increasing hardness can occur in a certain temperature range. Increasing the heating speed can move this temperature range to high temperature. It seems that this is related to the heterogeneity of austenite composition and the refinement of austenite grain and substructure during rapid heating.
(2) Wear resistance
After rapid heating and surface quenching, the wear resistance of the workpiece is higher than that of ordinary quenching. The wear resistance of rapid surface quenching is better than that of ordinary quenching. It seems that this is also related to austenite grain refinement, uneven austenite composition, high surface hardness and surface compressive stress state.
(3) Fatigue strength
Using correct surface quenching process can significantly improve the fatigue resistance of parts. For example, for 40gr steel, the fatigue limit of quenching and tempering plus surface quenching (hardening layer depth 0,9mm) is 324N/mm2, while that of withering treatment is only 235n/mm2. Surface quenching can also significantly reduce the notch sensitivity in fatigue test. The reason why surface quenching improves the fatigue strength is not only due to the increase of the strength of the surface itself, but also due to the formation of large residual compressive stress on the surface. The greater the surface residual compressive stress, the higher the fatigue resistance of the workpiece.

Influence of depth and distribution of surface hardening layer on workpiece bearing capacity

Although surface quenching has the above advantages, improper use will also bring the opposite effect. For example, improper selection of hardening layer depth or improper distribution of local surface hardening layer can cause stress concentration and failure in local places.
(1) The matching of the stress distribution between the surface hardening layer and the workpiece under load, that is, the depth of the surface hardening layer must match the load.
(2) Relationship between surface hardened layer depth and residual stress in workpiece
During surface quenching, only the surface is heated and only the surface expands and shrinks, so the surface will bear compressive stress. During quenching and cooling, the surface thermal stress is tensile stress, while the surface microstructure stress is compressive stress. As a result of the superposition of the two, the surface residual stress is compressive stress. This internal stress is caused by the expansion and contraction of the surface part during heating and cooling and the change of specific volume during microstructure transformation. It is obvious that the magnitude and distribution of the stress are related to the depth of the hardened layer. The test shows that when the workpiece diameter is certain, the surface residual compressive stress first increases with the thickening of the hardened layer depth. After reaching a certain value, if the hardened layer depth continues to be thickened, The surface residual compressive stress decreases. The residual stress is also related to the hardness distribution along the depth of quenching layer, that is, to the ratio between the depth of martensite layer, the width of transition zone and the cross-section size of workpiece.

What is induction hardening

Induction heating surface quenching refers to the induction current of a certain frequency generated in the workpiece by the heating inductor connected with AC. the skin effect of the induction current makes the workpiece surface layer quickly heated to the austenite area, and then spray water for cooling immediately, so as to obtain a certain depth of hardened layer on the workpiece surface. The higher the current frequency, the shallower the hardened layer.
Induction heating surface quenching is a quenching method that uses the principle of electromagnetic induction to make the parts cut the magnetic line of force in the alternating magnetic field, generate induced current on the surface, and quickly heat the part surface in the form of eddy current and then quench according to the alternating current skin effect. It plays an important role in the field of heat treatment, and this technology has been widely used in our country.
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Types of induction heating surface quenching

According to different current frequencies used, it can be divided into:

  • 1) High frequency induction heating surface quenching: the current frequency is 100-500khz, the most common frequency is 200-300khz, and the hardenable layer concentration is 0.5-2.0mm. It is mainly suitable for the surface quenching of medium and small module gears and medium and small size shaft parts.
  • 2) Medium frequency induction heating surface quenching: the current frequency is 500-10000hz, and the most common frequency is 2500-8000hz. The depth of hardenable layer is 3-5mm. It is mainly used for surface quenching of large-size shaft parts requiring deep hardening layer and large and medium modulus gears.
  • 3) Power frequency induction heating surface quenching: the current frequency is 50Hz, and frequency conversion equipment is not required. The depth of hardened layer is 10-15mm. It is suitable for surface quenching of large diameter parts such as rolls and train wheels.

Induction heating surface quenching VS ordinary heating quenching

Compared with ordinary heating quenching:

  • 1) Due to the extremely fast induction heating speed, the austenitizing temperature of steel increases significantly and the austenitizing time is significantly shortened, that is, austenitizing is completed in a wide temperature range in a short time.
  • 2) Due to the short induction heating time and high superheat, austenite nucleates more and is not easy to grow. Therefore, fine cryptocrystalline martensite is obtained on the surface after quenching, the hardness is 2-3HRC higher than that of ordinary quenching, and the toughness is also significantly improved.
  • 3) After surface quenching, not only the surface strength of the workpiece is high, but also the volume expansion caused by martensitic transformation causes favorable residual compressive stress on the surface of the workpiece, which effectively improves the fatigue strength of the workpiece and reduces the notch sensitivity.
  • 4) The induction heating speed is fast and the time is short. Generally, the workpiece will not undergo oxidation and decarburization; At the same time, the quenching deformation is small because the core is not heated.
  • 5) Induction heating surface quenching has high production efficiency and is convenient for practical mechanization and automation; However, due to the high cost of equipment, it is not suitable for single piece production.

Induction heating quenching is generally used for medium carbon and medium carbon low alloy structural steels, such as 45#, 40Cr, 40MnB, etc.

Physical process of induction heating

The eddy current with the maximum density in the workpiece section gradually moves from the surface to the center, and is heated from the surface to the center in turn. This heating method is called penetration heating.
When heating is continued, the electric energy becomes heat only within the range of hot current penetrating into the layer, and the temperature of this layer continues to rise. At the same time, due to the effect of heat conduction, heat is transferred to the interior of the workpiece, and the thickness of the heating layer is thickened. At this time, the heating inside the workpiece is the same as that of ordinary heating, which is called conductive heating.
Compared with conduction heating, penetration heating has the following characteristics:

  • 1) When the surface temperature exceeds A2, the maximum density eddy current moves to the inner layer, and the heating speed of the surface layer begins to slow down and is not easy to overheat, while the conduction heating is easy to overheat when the surface continues to heat with the extension of heating time;
  • 2) Rapid heating, small heat loss and high thermal efficiency;
  • 3) The heat distribution is steep and the transition layer is narrow after quenching, which increases the surface compressive stress.

Induction heating surface quenching process

Induction heating surface quenching is to control the thermal parameters by adjusting the electrical parameters of the equipment to ensure the surface quenching quality. Thermal parameters include induction heating temperature, heating time, heating speed, etc. electrical parameters include equipment frequency, unit surface power of parts, anode voltage, anode current, channel voltage, grid current, etc.
(1) Basis for determining process parameters of induction heating surface quenching
Selection of steel grade:

  • Obtain a hard and wear-resistant surface layer → select medium carbon steel or high carbon steel;
  • The hard surface and the strength and toughness of the core → alloy steel should be obtained, and the microstructure of fine carbides distributed on the ferrite matrix should be obtained after pre heat treatment;
  • Compared with the traditional heat treatment steel, the steel with lower alloy content can be used under the same conditions.

Carbon content:

  • The main factors determining the hardness of induction heating surface hardening steel;
  • When replacing carburized steel, due to the high surface hardness requirements, the steel with high carbon content (0.50-0.60%) should be selected;
  • In order to improve the wear resistance of quenched and tempered parts, the most suitable carbon content is 0.40-0.50%.

Original structure before pre heat treatment and induction heating surface quenching:

  • The original structure is a structure in which dispersed fine carbide particles are evenly distributed on the ferrite matrix. During induction heating, it can quickly change to a at a lower temperature and realize the homogenization of a in a short time. When the parts are made of this original structure steel, the surface layer with thin hardening layer and high hardness and good comprehensive mechanical properties of the core can be obtained.
  • Medium carbon steel with fine pearlite and a small amount of ferrite obtained by normalizing pre heat treatment can also obtain better results, but its mechanical properties are worse than those of tempered sorbite.
  • The mechanical properties of the core of medium carbon steel with coarse pearlite and ferrite or pearlite and large ferrite obtained by casting, forging, hot rolling or annealing are poor after induction rapid heating and surface quenching.
  • Coarse spheroidized structure or large ferrite structure has an adverse effect on induction heating surface quenching.

Surface hardness:
There are different requirements for parts with different materials working under different conditions.
Hardened layer depth:
The depth of hardened layer specified on the drawing is determined to ensure better mechanical properties of parts. In fact, the size and distribution of transition layer after induction heating surface quenching shall also be controlled. (hardened layer refers to the distance from all m on the surface to half m; transition layer refers to the distance from half m to the appearance of original tissue;)
The transition layer presents a state of tensile stress. If it is too wide, the surface compressive stress will be reduced, resulting in the reduction of fatigue strength. The transition layer is generally 25-30% of the hardened layer depth.
Hardened area:

  • In order to prevent stress concentration, quenching deformation and cracking in the quenched transition layer of parts, reasonable provisions shall be made for the depth of hardened layer and the position of hardened zone.

Deformation – deformation of hollow cylindrical or annular parts:

  • The depth of hardened layer is less than 0.35 × (outer ring diameter – inner ring diameter), deformation law: quench the outer circle, and the outer diameter expands; Quench the inner circle and reduce the inner diameter.
  • The depth of hardened layer is greater than 0.1 × (cylindrical outer diameter or outer ring diameter – cylindrical inner diameter or inner ring diameter), deformation law: quench the outer circle, the outer diameter increases and the inner diameter decreases; Quench the inner circle, reduce the inner diameter and expand the outer diameter.

(2) Determination of induction heating surface quenching process parameters
Select the equipment reasonably according to the requirements of part size and hardened layer depth.
① Selection of equipment frequency:

  • It is mainly selected according to the depth of the hardened layer. After the equipment is determined, the frequency cannot be adjusted arbitrarily, and the penetration depth of the current frequency cannot be selected arbitrarily according to the requirements of the depth of the hardened layer. To realize penetration heating, the selection range of current frequency is 150/ δ 2<f<2500/ δ 2;
  • When the depth of hardened layer is known, the best current frequency can be found. Practice has proved that when the hardened layer depth is half of the current heat penetration depth, the best current frequency can be obtained: fbest = 600/δ 2.

Hardened layer depth

Current frequency/Hz

mm

Highest

Optimum

Minimum

1

250000

60000

15000

2

100000

25000

7000

3

30000

7000

1500

4

15000

4000

1000

6

8000

1500

500

10

2500

500

150

When the depth of the hardened layer is required to be greater than the current penetration depth that can be achieved by the frequency of the existing equipment, the method adopted under the condition of ensuring that the surface is not overheated:

  • Reduce the input power per unit surface of parts and prolong the heating time;
  • Increase the gap between the parts and the inductor and prolong the heating time; Or intermittent heating method is adopted when heating at the same time to increase the heat conduction time;
  • Parts shall be preheated in the inductor before induction heating;
  • For continuous heating, double turn or multi turn inductors are used;
  • Use smaller unit surface power to prolong heating time;
  • When the part size is large and the equipment power is insufficient, continuous sequential heating and quenching shall be adopted to minimize the surface area heated by the inductor, so as to improve the unit surface power, and preheating measures shall be taken at the same time.

② Selection of specific power:
Specific power refers to the electric power absorbed on the unit surface area of the workpiece during induction heating; When the frequency is constant, the higher the specific power is, the faster the heating speed is; When the specific power is constant, the higher the frequency is, the shallower the current enters and the faster the heating speed is. The selection of specific power mainly depends on the frequency and the required hardened layer depth. When the frequency is constant and the hardened layer is shallow, the higher specific power (penetration heating) shall be selected; Under the condition of the same layer depth, the equipment with lower frequency can choose higher specific power.
(3) Selection of quenching heating temperature and mode
The selection of induction heating quenching temperature is related to the heating speed and the original structure before quenching.
Higher quenching heating temperature is adopted. Generally, the quenching temperature of high-frequency heating can be 30 ~ 200 ℃ higher than that of ordinary heating and quenching. If the heating speed is fast, a higher temperature shall be adopted; If the original structure before quenching is different, the quenching heating temperature can also be adjusted appropriately. The microstructure of quenching and tempering treatment is more uniform than that of normalizing, and lower temperature can be used.
The heating methods include simultaneous heating method and continuous heating method.
Simultaneous heating method: it is generally used when the equipment power is sufficient and the production batch is relatively large.
Continuous heating method: part of the parts of the workpiece to be quenched shall be heated at the same time. Through the relative movement between the inductor and the workpiece, the heated parts shall be gradually moved to the cooling position for cooling, and the parts to be heated shall be moved to the inductor for heating. This process shall be carried out continuously until all parts to be hardened are quenched.
The cooling methods include spray cooling method and immersion quenching method.
Spray cooling method means that when induction heating is completed, the workpiece is placed in the ejector and sprayed with quenching medium to quench and cool the workpiece. Immersion quenching means that the workpiece is immersed in quenching medium for cooling at the end of heating.
For fine and thin workpieces or alloy steel gears, in order to reduce deformation and cracking, the inductor and workpieces can be put into the oil tank for heating and cooling after power failure. This method is called buried oil quenching method.
The quenching medium includes water, polyvinyl alcohol aqueous solution, polyacrylic alcohol aqueous solution, emulsion and oil.
Water can be used for spray cooling. It is suitable for parts with simple shape made of low hardenability steel.
Polyvinyl alcohol aqueous solution is used for jet cooling, and its cooling capacity decreases with the increase of concentration. The commonly used concentration is 0.05-0.30%; If the concentration is 0.3%, the best use temperature is 32-43 ℃, which should not be lower than 15 ℃.
Emulsion (3%) is used for spray cooling, parts made of low alloy steel and parts made of carbon steel with complex shape to prevent cracking of these parts;
Oil is used for immersion cooling of alloy steel. When in use, it must have good ventilation and fire extinguishing conditions; It can also be used for jet cooling. When buried oil quenching is adopted, oil injection device or oil agitator is generally set in the oil to forcibly cool the oil.
(4) Tempering process
Low temperature tempering is generally used to reduce residual stress and brittleness without reducing hardness.
Tempering methods generally include furnace tempering, self tempering and induction heating tempering. Self tempering refers to tempering with the residual heat in the workpiece when it has not been completely cooled after quenching.
Tempering temperature in the furnace is 150-180 ℃ for 1-2 hours.
The self tempering temperature is about 80 ℃ higher than that in the furnace, and the self tempering time is short.
Induction heating tempering is to reduce the tensile stress of the transition layer. The type of heat source shall be medium frequency or power frequency heating and tempering to ensure that the depth of the heating layer is deeper than that of the hardened layer. Induction heating tempering has shorter heating time than tempering in furnace, large carbide dispersion in microstructure → steel parts have high wear resistance and good impact toughness. The heating speed is generally less than 15 – 20 ℃/s.

What is flame heated surface quenching

Flame heating surface quenching is a process of austenitizing and quenching a workpiece surface by heating it with a flame in several size ranges.
Advantages of flame heating surface quenching: simple equipment, convenient use and low cost; It is not limited by the volume of the workpiece and can be moved and used flexibly; After quenching, the surface is clean without oxidation and decarburization, and the deformation is small.
Disadvantages of flame heating surface quenching: the surface is easy to overheat; It is difficult to obtain the hardened layer depth less than 2mm, which is only suitable for the surface layer with convenient flame spraying; The gas mixture used is explosive.

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Flame heating quenching method

(1) Simultaneous heating and quenching
Simultaneous heating quenching heats the surface of the quenched workpiece to the quenching temperature at one time, and then sprays water or immerses it in the quenching medium for cooling.
At the same time, heating quenching belongs to surface quenching with small area, which is suitable for mass production and easy to realize automation.
(2) Rotary frame quenching method
Rotary frame quenching method rotates the workpiece in the process of heating and cooling, which can heat the workpiece evenly.
The rotary quenching method is applicable to the surface quenching of cylindrical or disk-shaped workpieces.
(3) Swing quenching method
Swing quenching method refers to swinging back and forth on the workpiece by the nozzle to expand the heating area. When the surface of the part to be heated reaches the heating temperature evenly, it is cooled by the same method as the simultaneous heating method.
The swing quenching method is suitable for surface quenching with large area and deep hardening layer.
(4) Propulsion quenching method
The advancing quenching method refers to that the nozzle continuously advances and heats along the parts to be quenched on the workpiece surface, and the water sprayer then cools and quenches with water spray.
The propulsion quenching method is applicable to the quenching of guide rails, sliding grooves of machine tool bed, etc.
(5) Rotary continuous quenching
Rotary continuous quenching method refers to the combination of rotary quenching and propulsion quenching.
Rotary continuous quenching method is suitable for surface quenching of shaft parts.
(6) Peripheral continuous quenching
Rotary continuous quenching means that the flame nozzle and water sprayer make a curve movement along the periphery of the quenched workpiece to heat and cool the periphery of the workpiece.
The characteristic of rotary continuous quenching method is to produce soft bands when the initial quenching zone meets the final quenching heating zone.

Characteristics of flame heating quenching process

For flame quenching with specified hardening layer depth, the heating temperature of workpiece surface shall be 20-30 ℃ higher than that of ordinary quenching.
Due to the fast heating speed, it is best to normalize or quench and temper the workpiece to obtain fine sheet P
When the heating depth is large, preheating shall be carried out to prevent workpiece cracking.
The distance between the nozzle and the workpiece shall be kept in the area with the highest thermal efficiency, generally 2-3mm from the top of the reduction area to the workpiece surface.
If the nozzle moves slowly, the workpiece surface will overheat and the hardness will decrease after quenching.
The faster the moving speed of the nozzle and the smaller the gas flow, the thinner the hardening layer depth.
The interval between heating stop and water spray cooling should be 5-6s.
During continuous heating and quenching, the distance between the flame nozzle and the water spray hole shall be controlled within 10-15mm.
Manual flame quenching can put the workpiece into water or oil for cooling. It is suitable for small alloy steel or carbon steel parts that do not need quenching. If quench is required, spray holes can be machined on the nozzle to spray cooling medium for continuous cooling.
The rotating method can spray cooling medium on the cooling ring.
Tempering in furnace or self tempering after flame quenching. The tempering temperature in the furnace is 180-220 ℃, and the holding time is 1-2h.

What is laser surface quenching

Laser surface quenching will apply104~105W/cm2 high power density laser beam on the workpiece surface, rapidly raise the temperature of the workpiece surface above the phase transformation point at the heating speed of 105~106℃/s, and then self cooling quenching at the speed of 105℃/s by relying on the cold matrix.

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Principle of laser quenching

Laser quenching technology is to use the focused laser beam to shoot into the surface of steel materials to rapidly raise the temperature above the transformation point. After the laser is removed, due to the rapid thermal conduction of the inner material still at low temperature, the heated surface is quickly cooled below the martensitic transformation point, so as to realize the surface transformation hardening of the workpiece. For example, the maximum hardened layer depth of laser melting quenching on the surface of large roll can reach more than 2mm. It has the characteristics of fast heating speed, fine structure, high hardenability and no deformation, and has wide technical applicability, which is not limited by the manufacturing difficulty of the inductor.

Characteristics of laser quenching

Compared with the existing medium and high frequency quenching and carburizing quenching in the factory, laser quenching has the following characteristics:

  • The power density is high, the heating speed is very fast, and the deformation of parts is very small.
  • Parts with complex shapes can be processed or partially processed, or different processing can be carried out in different parts of the same part according to needs.
  • It has strong universality.
  • For some stainless steel parts with high quenching temperature, the quenching temperature is very close to the melting point temperature. When using the inductor for local surface quenching of the product, it is easy to burn the included angle or irregular parts, resulting in the scrapping of the parts, while laser surface quenching is not limited.
  • The cooling speed of laser quenching is fast.
  • The surface hardening layer has fine structure, high hardness and good wear resistance, which can meet the surface quenching products with shallow hardening layer depth (generally 0.3 – 2.0mm).

Application scope of laser quenching

The depth of hardened layer is ≤ 0.75mm, the width is < 1.2mm, the surface hardening efficiency is 80 – 85mm2/min, and the general laser surface quenching power is 1 – 6kW/cm2.

Process parameters and their relationship

Laser surface quenching is a complicated quenching process of rapid heating and cooling. The dimensional parameters (hardening layer width, hardening layer depth, surface roughness) and performance parameters (surface hardness, wear resistance, microstructure change) of the laser hardening layer depend on the laser power density (laser power, spot size), scanning speed, material characteristics (composition, original state) and material surface pretreatment, It is also related to the geometry and size of the processed parts and the thermodynamic properties of the laser action zone.
In addition, the selection range of various parameter values shall be considered. D shall not be too large and V shall not be too small, so as to avoid too low cooling rate and failure to realize martensitic transformation. On the contrary, when the laser output power is too large, it is easy to cause surface melting and affect the geometry of the surface.

Scanning mode of laser quenching

The scanning methods of laser quenching include narrow-band scanning of circular or rectangular spot and wide-band scanning of linear spot.

Microstructure and properties of laser hardened layer

The laser quenched microstructure is divided into transformation hardening zone, transition zone and matrix.
The transformation hardening zone is very fine martensite; The transition zone is a complex multiphase structure; The matrix is the original matrix structure.
Hardness of laser hardened layer:

  • The laser surface quenched sample is cut along the scanning center band to prepare the metallographic sample. Carry out hardness test with microhardness tester (load: 200gf, holding time: 10s).
  • The hardness of laser hardened layer is 15% – 20% higher than that of conventional quenching due to extremely rapid heating and cooling. The hardness of hardened layer is related to the hardenability of steel.

Advantages of laser quenching

  • The microstructure of the hardened layer is refined, the hardness is 15% – 20% higher than that of conventional quenching, and the wear resistance is increased by 1 – 10 times.
  • It can accurately control the depth of hardened layer, small workpiece deformation and no oxidation and decarburization on the surface.
  • Local selective quenching can be carried out, and surface hardening can be realized as long as the parts that can be irradiated by laser. (local selective heating and quenching can be carried out according to any complex geometric figure without affecting the organization and finish of adjacent parts. It is convenient to heat and quench some corners, narrow grooves, racks, gears, deep holes, blind hole surfaces, etc. with optical conduction system and mirror)
  • Fast heating speed, high degree of automation, high production efficiency and almost no deformation.

Disadvantages of laser quenching

  • The workpiece surface needs to be pretreated to increase the ability of the workpiece to absorb laser.
  • The equipment is expensive.
  • Fast heating speed, quenching without coolant.

What is electron beam heated surface quenching

Electron beam heating surface quenching bombards the metal surface by electron flow, and the electron flow collides with atoms in the metal to transfer energy for heating. When the electron beam bombards the surface in a short time, the surface temperature increases rapidly, while the substrate remains cold. When the electron beam stops bombarding, the heat is rapidly transmitted to the cold base metal, so that the heated surface is quenched by itself.

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Characteristics of electron beam heating surface quenching

  • High heating efficiency and minimum energy consumption;
  • A certain degree of vacuum is required;
  • Heating the workpiece surface without special treatment;
  • The controllability is worse than that of laser;
  • In the initial stage of heating, high power density and small beam spot diameter are used to obtain high power density heating; In the later stage of heating, low power density and large beam spot diameter are adopted to reduce power, obtain a heating layer with higher temperature in a certain depth range, and improve the depth of hardening layer.

Compared with the hardness obtained by electron beam heating surface quenching and conventional quenching, it can be seen that the hardness value is increased by 3 – 4 HRC. The microstructure of the surface quenched by electron beam is the hardened layer on the surface and the tempering area between the hardened layer and the core. The hardened layer is a region with heating temperature higher than AC3. After rapid cooling, it changes into martensite. Its microstructure is low-carbon lath martensite or fine needle martensite plus uniformly distributed carbides. The structure of the tempering zone depends on the heat treatment before electron beam hardening. Because the temperature is lower than the phase transformation point, the matrix structure may recover or recrystallize, or may contain ferrite grains; Tempering can also occur in the area where the electron beam overlaps, which is beneficial to improve the quality of the hardened layer. The hardened layer depth of electron beam surface quenching is generally from a few microns to a few millimeters. Due to the improvement of surface hardness, the friction properties of steel after electron beam surface quenching can be greatly improved, and the fatigue properties can also be improved.
Electron beam surface quenching is applicable to low carbon steel, alloy structural steel, bearing steel and tool steel. In addition, white iron and gray cast iron can also be treated by this technology. The combination of electron beam surface quenching technology and ion nitriding technology can further improve the surface hardness and wear resistance.
According to the microhardness distribution and wear resistance curve of the surface layer before and after electron beam quenching after nitriding, it can be seen that the microhardness in the hardened layer has been greatly improved, the high hardness area has been significantly deepened, and the wear resistance has been doubled.

What is plasma beam surface quenching

Plasma surface quenching is the application of plasma beam to heat the surface of metal materials above the transformation point. With the cooling of the material itself, austenite is transformed into martensite, and a hardening belt composed of ultra-fine martensite is formed on the surface. It has higher surface hardness and strengthening effect than conventional quenching. At the same time, considerable compressive stress remains in the hardened layer, which increases the fatigue strength of the surface.
Using this feature to implement plasma quenching on the surface of parts can improve the wear resistance and fatigue resistance of materials. Moreover, due to the fast plasma surface quenching speed and less heat entering the workpiece, the resulting thermal distortion is small (the distortion variable is 1/3-1/10 of high-frequency quenching). Therefore, the workload of the subsequent process (correction or grinding) can be reduced and the manufacturing cost of the workpiece can be reduced. In addition, the process is self cooling, which is a clean and sanitary heat treatment method.

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The research shows that the treatment of typical parts of cast iron, carbon steel and alloy steel by plasma surface quenching can significantly improve their service performance and prolong their service life, such as cylinder liner and rocker arm parts of internal combustion engine, coreless raceway of automobile trailer, lead screw of plastic spraying machine, tooling, machine bed guide rail, roll of heat exchanger production line and so on.
In recent years, some new progress has been made in the field of plasma surface quenching, among which the more prominent are the research on the cross-section power density distribution of plasma beam for quenching and multi-element co alloying while plasma beam scanning quenching.
The research on the friction and wear mechanism at high temperature and high speed shows that the iron carbon martensite obtained by simple surface quenching will produce instantaneous surface micro protrusion, contact point will be softened by high temperature annealing under the condition of high temperature and high speed friction, increase the friction coefficient, reduce the wear resistance and matching performance, and lead to the increase of hardness nearly twice after surface quenching, However, the wear life is less than doubled. However, alloy martensite has high tempering resistance, that is, it has high red hardness. Under the instantaneous friction and high temperature on the surface, the contact point does not soften. At the same time, different alloy elements have the characteristics of anti adhesive wear, which can enhance their wear resistance, reduce the friction coefficient and improve the matching performance, Therefore, surface alloying element Co infiltration and quenching strengthening is the most reasonable and effective way to resist high-temperature and high-speed wear and reduce friction coefficient. Rapid plasma beam scanning and simultaneous infiltration and quenching of surface alloying elements are valuable practical technologies for micro deformation, high-quality, efficient and low-cost surface strengthening of such friction pair parts, The technology has been widely used in the cylinder liner of internal combustion engine, and good results have been obtained.

Characteristics of plasma beam surface quenching

  • 1) Simple preparation and less investment. The plasma beam gun generator mainly includes the power supply for generating arc discharge and the transmitting gun for generating plasma beam, and its investment is less than 1/3 of that of the laser generator.
  • 2) Before laser beam heating, the surface of the heated workpiece needs blackening treatment, electron beam heating surface quenching needs to be carried out in the vacuum chamber, while plasma beam heating does not need blackening treatment or in the vacuum chamber.
  • 3) It can handle workpieces with special-shaped section. Because the plasma beam gun can move freely, probe into the interior of the workpiece and heat the inner surface of the workpiece, such as the inner cylindrical surface, groove and other surfaces. If these surfaces are heated by laser beam, a laser reflection device needs to be added, while the electron beam cannot be heated.
  • 4) Plasma beam heating can be carried out under the protection of inert gas, so there is no oxidative decarburization. If the reaction gas is used as the carrier gas, such as nitrogen, nitrogen can also be doped during quenching and heating to form nitrides on the surface, so as to further improve the hardness and wear resistance of the hardened layer.
  • 5) Fast heating speed and high treatment efficiency.
  • 6) The treated surface has no oxidation and high quality.
  • 7) It is not suitable for the whole plane hardening treatment.

Source: Network Arrangement – China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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