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Summary of quenching quality defects and control

Steel after quenching can increase its strength, hardness and wear resistance, but the original size or shape of workpiece during quenching happens when people don’t want to change, this change will be affect the quality of the product defects and reduce or avoid these shortcomings, first of all, we will know what defect of the quenching will be what is its forming reason, and find the corresponding solution. Quenching quality defects and control are described from the following aspects.

20211015021322 10161 - Summary of quenching quality defects and control

Quenching distortion

Quenching distortion can be divided into two types: volume distortion and shape distortion.

The main reason for the volume change is the different specific volumes of different tissues before and after quenching. The specific volume of martensite → bainite → pearlite → austenite decreases in turn. The original pearlite structure of the workpiece quenching into martensite, volume expansion. If the tissue has a large amount of residual austenite, it may be reduced in volume. Only especially high precision of the workpiece to consider the volume of uniform expansion caused by volume size change.

Changes in the relative position or size of each part of the workpiece, such as plate and rod bending, expansion and contraction of the inner hole, hole spacing changes, are collectively referred to as shape distortion. The causes of distortion are as follows:

  • (1) The heating temperature is not uniform, the thermal stress caused by distortion or the workpiece is placed in the furnace is not reasonable, at high temperature often due to the creep distortion of the dead weight.
  • (2) During heating, the yield strength of steel decreases with the increase of heating temperature. When the residual stress (cold deformation stress, welding stress, machining stress, etc.) in the workpiece reaches the yield strength at high temperature, it will cause uneven plastic deformation of the workpiece and cause shape distortion and residual stress relaxation.
  • (3) The thermal stress and microstructure stress formed by the difference of quenching cooling make the workpiece local plastic deformation. The workpiece of complex shape, because of the particularity of its structure, when quenching, because of the speed of heating and cooling is not the same, increase its deformation tendency.

Ways and methods to reduce quenching distortion

  • (1) Using reasonable heat treatment process can effectively reduce the distortion. Such as reducing the quenching heating temperature; Slowly heating or preheating the workpiece; Static heating method, very slender and very thin workpiece, in order to reduce the impact of salt bath magnetic stirring on the workpiece, can use power heating; Small section size of the workpiece, if the strength of the center is not high, the use of rapid heating; Reasonable strapping and hanging of the workpiece; According to the shape of the work using a reasonable way of quenching; Step quenching or isothermal quenching; According to the shape characteristics and deformation law of the workpiece, the workpiece is inverted deformation before quenching, so that it is offset with the distortion after quenching.
  • (2) Reasonable design of parts. Such as the workpiece shape strive to symmetry, avoid the section gap, so as to reduce the distortion caused by uneven cooling; Easy to distortion of the groove workpiece or open workpiece, in order to reduce the expansion or contraction of the notch, before quenching to make it a closed structure, such as the reinforcement in the notch, quenching and then cut; Process holes are arranged to reduce the cavity shrinkage; The complex part adopts composite structure, that is, a complex workpiece is decomposed into several simple parts, and then assembled after micro-distortion quenching. Correct selection of steel, such as high precision, allow heat treatment of small distortion of the mold, can choose micro distortion steel, high precision plastic mold can also choose pre-hard steel.
  • (3) Reasonable forging and preparatory heat treatment. The severe carbide segregation and banded structure make the quenching distortion anisotropic or irregular. Improving the carbide distribution by forging can not only reduce the distortion, but also prolong the service life of the workpiece.

Distortion correction

For the distortion of heat-treated parts, cold pressing straightening, hot spot straightening, hot straightening, tempering straightening, counterattack straightening, shrinkage cavity treatment and other methods can be adopted.

Cold pressing straightening is to bend the workpiece at the highest point of the convex force, so that the plastic deformation, this method is suitable for the hardness of less than 35HRC axis workpiece; Hot spot straightening is to heat the convex part with oxygen acetylene flame, and then cool it quickly with water or oil, so that the heated part shrinks under the action of thermal stress. This method is suitable for the workpiece with hardness greater than 35 ~ 40HRC; Straightening while hot is to harden the workpiece to near Ms temperature, using the good plasticity of austenite and transformation superplasticity, so that the distortion is corrected; Tempering correction is to apply external force to the workpiece, and then tempering, tempering temperature is higher than 300℃; Counterattack straightening is to use a steel hammer to continuously hit the concave, so that the small area of the workpiece plastic deformation; Shrinkage cavity treatment is to quench the swelling workpiece, heated to 600 ~ 700℃ through red, in order to prevent water into the hole, with two sheets to cover both ends of the workpiece, quickly put into water quenching, using thermal stress to make the hole shrink, after one or multiple repeated operation, can make the swelling hole correction.

Quenching cracking

Quenching cracking is the cracking phenomenon caused by heat treatment stress exceeding the fracture strength of materials. The cracks were distributed in intermittent series, with traces of quenching oil or salt water on the fracture, without oxidizing color, and without decarbonization on both sides of the crack. The occasions and causes of quenching cracks are as follows:

  • (1) Material management is chaotic, mistaking high carbon steel or high carbon alloy steel for low and medium carbon steel and using water quenching.
  • (2) Improper cooling. Fast cooling below Ms temperature, cracking due to high stress in the structure. Such as water-oil dual medium quenching, the residence time in water is long, quenching oil contains too much water.
  • (3) When the hardness of the core of the unquenched workpiece is 36-45HRC, a quench crack is formed at the junction of the quenched layer and the non-quenched layer. When the core hardness is less than 36HRC, the tensile force at the junction is reduced. When the core hardness is greater than 45HRC, it indicates that there is martensitic structure, tensile stress peak decreases, and cracking tendency decreases.
  • (4) The workpiece with the most dangerous quenching crack size is easy to form quenching crack. When the workpiece is completely quenched, one of the most dangerous quenching size, its diameter is: 8 ~ 15mm when water quenching; 25 mm to 40mm in oil quenching. When the size is smaller than the most dangerous quenching size, the temperature difference between the core and the surface is small, the hardening force is small, and it is not easy to crack, but increases, but the tensile stress peak is far away from the surface, and the quenching tendency decreases.
  • (5) Serious surface decarbonization is easy to form network cracks. The martensite of the decarburized layer is small in volume and easy to form network cracks under tensile stress.
  • (6) For deep-hole workpieces with small inner diameter, the cooling of the inner surface is much smaller than that of the outer surface, the residual thermal stress is small, and the residual tensile stress is larger than that of the outer surface, and the inner wall is easy to form parallel longitudinal cracks.
  • (7) Quenching heating temperature is too high, causing grain coarsening, grain boundary weakening, steel brittle fracture strength reduction, quenching easy cracking.
  • (8) Without intermediate annealing before repeated quenching, the tendency of overheating is large, the quenching stress in the preceding paragraph is not completely eliminated, and the surface decarbonization caused by multiple heating will promote quenching cracking.
  • (9) Large section high alloy steel workpiece quenching heating without preheating or heating speed is too fast, heating thermal stress or microstructure stress increases, causing cracking.
  • (10) The original structure is poor, such as high carbon steel spheroidizing annealing quality is poor, its structure is flake or point pearlite, thermal tendency is large; Grain coarsening, high martensite content, quenching cracking tendency.
  • (11) The microcracks of raw materials, non-metallic inclusions, and the tendency of severe carbide segregation quenching cracking increases. Such as non-metal plus sundry or heavy carbide along the rolling direction of banded release, due to mechanical properties anisotropy, its transverse performance is 30% ~ 50% lower than the longitudinal performance, under the maximum surface tensile stress, often along the non-metal inclusion or carbide distribution direction of longitudinal crack.
  • (12) The forging crack expands during quenching. During quenching heating in flexible furnace, there are black oxide skin on the cracked section and decarburization layer on both sides of the crack.
  • (13) Overfiring crack. The cracks are usually reticulated and the grain boundary is oxidized and melted.
  • (14) When steel with low hardenability is clamped and quenched with pliers, the quenched part is cooled slowly and has non-martensitic structure. The clamp mouth is located at the junction of hardened layer and non-hardened layer, and the tensile stress is large and easy to crack.
  • (15) High speed steel, high chromium steel grading quenching, the workpiece is not cold to room temperature, eager to clean (due to rapid cooling below Ms) caused by cracking.
  • (16) The thermal stress and microstructure stress formed by cryogenic treatment are relatively large due to rapid cooling and rapid heating, and the brittle fracture strength of low-temperature materials is low, which is easy to produce quenching cracking.
  • (17) Without timely tempering after quenching, the microscopic cracks inside the workpiece expand and form macroscopic cracks under the action of quenching stress.

Measures to prevent quenching cracking

  • (1) Improve the structure of the workpiece. The section should be uniform, different sections should have rounded corner transition, as far as possible to reduce the hole, sharp Angle, to avoid stress concentration caused by cracking.
  • (2) Reasonable selection of steel. Alloy steel with high hardenability should be selected for the workpiece with complex shape and easy cracking, so as to use the quenching medium with slow cooling rate and reduce the quenching stress.
  • (3) The raw materials should avoid microcracks and serious non-metallic inclusions and carbide segregation.
  • (4) Correct pre-heat treatment to avoid normalizing and annealing defects.
  • (5) Correctly select heating parameters.
  • (6) Reasonable selection of quenching medium and quenching method.
  • (7) The parts of the workpiece prone to cracking, such as sharp corners, thin walls, holes and so on for local bandaging.
  • (8) Cracking workpiece after quenching should be tempered in time or with temperature tempering.

Insufficient hardness

The surface hardness of the workpiece after quenching is lower than the hardening hardness value of the steel used is called insufficient hardness. There are many reasons for insufficient hardness, and the control measures are shown in Table 1.

Table 1

The serial number

Cause of insufficient hardening hardness

Control measures

1

The cooling ability of the medium is poor, and the surface of the workpiece has ferrite, tortensite and other non-martensite structures.

Adopt quenching medium with faster cooling; Increase the quenching heating temperature appropriately.

2

The quenching heating temperature is low, or the pre-cooling time is long, the quenching cooling speed is low, and the non-martensitic structure appears.

Under the premise of ensuring normal quenching heating temperature; Reduce precooling time.

3

Hypoeutectoid steel underheated has undissolved ferrite.

Strictly control heating temperature, moisturizing time and furnace temperature uniformity.

4

Carbon steel or low alloy steel using water-oil dual medium quenching, the residence time in water is insufficient, or from the water parts, the residence time in the air is too long.

Strictly control the retention time of parts in water and operation standard.

5

Steel hardenability is poor, and the size of the working section is not large, can not be hardened.

Use steel with good hardenability.

6

The high carbon high alloy steel has excessive residual austenite due to high quenching temperature.

Reduce the quenching heating temperature or adopt cryogenic treatment.

7

Too long isothermal time causes austenite stabilization.

Strictly control grading or isothermal time.

8

Surface decarburization.

Use controlled atmosphere heating or other anti-decarbonization measures.

9

The water content in saltpeter or alkali bath is too little, and non-martensite such as tortensite is formed when graded cooling.

Strictly control water in salt bath and alkali bath.

10

Internal oxidation of alloying elements, reduced surface hardenability, the emergence of tortensite and other non-martensite and internal martensite structure.

Reduce oxidizing component content in furnace atmosphere; Choose quenching medium with fast cooling speed.

The soft point

After quenching, the phenomenon of low hardness in local areas of the workpiece surface is called soft spot. Carbon steel and low alloy steel are usually prone to soft spot due to poor hardenability. Causes of soft spots and control measures are shown in Table 2.

Table 2.

The serial number

Cause of soft point opening

Control measures

1

During quenching, the bubbles on the surface of the workpiece did not burst in time, resulting in the cooling rate of the bubbles reduced, and the non-martensitic structure appeared.

Increase the relative movement of medium and workpiece; Control water temperature and impurities in the water.

2

Local oxide scale, rust or other attachments (coating) on the working surface do not peel off when quenching, so that the cooling rate is reduced.

Clean the workpiece surface before quenching.

3

The original structure is not uniform, with severe zonal structure or carbide segregation.

The raw materials are forged and pre-heat treated to homogenize the structure.

Surface corrosion — pitting

After quenching the workpiece by pickling or sandblasting, the surface of the larger density of the pitted pits known as pitting, it is formed by the corrosion of the medium, pitting the workpiece luster, affect the surface smooth degree.

There are many reasons for the formation of pitting, but we can reduce this defect in the work, such as reducing the sulfate content in the salt bath, to avoid corrosion of the matrix; Can also reduce the nitrate temperature; The workpiece heated by high temperature quenching is pre-cooled and then put into the solution to avoid the decomposition of saltpeter. During local heating at high temperature, salt immersion treatment is carried out on the non-heated part to cover it with a solid salt shell to prevent pitting.

Authors: Cheng Zhanbei, Zhao Yongli, Yuan Youxi

Source: Network Arrangement – China Pipe Fitting 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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