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Discussion on forming and heat treatment process of duplex steel tee

Duplex stainless steel has good corrosion resistance and is widely used in petroleum, coal chemical industry and other fields, but the forming of pipe fittings of duplex steel (especially the forming of TEE) is easy to crack. Through the discussion of the cracking mechanism of duplex steel and the study of the law of thermal deformation, this paper finally solves the problem of forming cracking of duplex steel tee; At the same time, through the study of phase diagram, it is proposed to change the double ratio of dual phase steel by heat treatment process, so as to obtain good mechanical properties.

20220220110540 87185 - Discussion on forming and heat treatment process of duplex steel tee

1. What is duplex steel

Duplex steel as the name implies is duplex stainless steel, refers to the ferrite and austenite each about 50%, generally less phase content needs to reach a minimum of 30% of stainless steel. In the case of lower C content, Cr content in 18% to 28%, Ni content in 3% to 10%. Some steels also contain Mo, Cu, Nb, Ti, N and other alloying elements. This type of steel has both austenitic and ferritic stainless steel characteristics, yield strength up to 400Mpa – 550MPa, is two times the ordinary austenitic stainless steel. Compared with ferritic stainless steel, plasticity, toughness is higher, no room temperature embrittlement, intergranular corrosion resistance and welding performance are significantly improved, while also maintaining the ferrite stainless steel 475 ℃ brittleness and high thermal conductivity, with super plasticity and other characteristics. Compared with austenitic stainless steel, high strength and resistance to intergranular corrosion and chloride stress corrosion has been significantly improved. Duplex stainless steel has excellent resistance to pore corrosion and is also a nickel saving stainless steel.
Duplex stainless steel has the following performance characteristics.

  • 1) Molybdenum-containing duplex stainless steel has good resistance to chloride stress corrosion under low stress.
  • 2) Molybdenum-containing duplex stainless steel has good resistance to pore corrosion.
  • 3) Good corrosion fatigue and wear corrosion resistance.
  • 4) Good overall mechanical properties. Have high strength and fatigue strength, yield strength is 18-8 type austenitic stainless steel twice; solid solution state elongation of 25%, toughness value AK (V-notch) in more than 100J;.
  • 5) Good weldability, low thermal cracking tendency, generally no preheating before welding, no heat treatment after welding, can be welded with 18-8 austenitic stainless steel or carbon steel and other dissimilar species.
  • 6) Duplex stainless steel containing low chromium (18% Cr) hot working temperature range than 18-8 type austenitic stainless steel wide, small resistance, without forging, direct rolling open billet production of steel plates.
  • 7) Cold processing than 18-8 austenitic stainless steel process hardening effect is large, in the tube, plate to withstand deformation at the beginning, need to apply greater stress to deformation;
  • 8) Compared with austenitic stainless steel, the thermal conductivity is large, the coefficient of linear expansion is small, suitable for use as the lining of equipment and the production of composite plates. Also suitable for making the core of heat exchangers, heat transfer efficiency is higher than austenitic stainless steel; and so on.

It is due to the above advantages, duplex stainless steel is widely used in refining, fertilizer, paper, petroleum, chemical and other seawater resistant high temperature resistant concentrated nitric acid and other heat exchangers and cold shower and piping, pipe fittings.
Due to the characteristics of the two-phase organization, through the correct control of chemical composition and heat treatment process, so that the duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel, which combines the excellent toughness and weldability of austenitic stainless steel with the higher strength and chloride stress corrosion resistance of ferritic stainless steel, it is these superior properties make duplex stainless steel as a weldable structural material is developing rapidly.

2. Forming of duplex steel tee

With the wide application of duplex stainless steel, duplex stainless steel fittings also began to be used in large quantities, however, the production of pipe fittings encountered the following problems (pipe fittings forming to tee is the most complex, the following tee forming as a typical, to be described).

20220220110733 53681 - Discussion on forming and heat treatment process of duplex steel tee

Duplex stainless steel in the process of making a tee, there is generally a repeated heating and deformation forming procedure, we encountered cracking of the tee branch during the pressing process when pressing the tee, and the cracking is not accidental, it is frequent, therefore, in order to improve the yield and product quality, it is necessary to find out the causes and mechanisms of duplex stainless steel cracking, and finally come up with the technical parameters of the process to solve the problem.
Duplex stainless steel corrosion resistance depends largely on the double compared to the actual production, on the one hand, because the phase ratio of the raw material itself may not meet the standard requirements, on the other hand, the phase ratio of the product is changed due to repeated heating in the tee forming process, so it is necessary to restore the phase ratio of duplex stainless steel by means of heat treatment. However, the standard provides a wide range of solid melting temperature, in this solid melting temperature range, due to differences in the chemical composition of the material may lead to the phase ratio does not meet the standard requirements, so it is also necessary to find out the relationship between the solid melting temperature, chemical composition and phase ratio, the factors affecting these three parameters, and finally come up with the best heat treatment process specifications for duplex stainless steel.
Duplex steel deformation at high temperatures is prone to cracking. This is mainly in the high temperature ferrite phase is soft, austenite phase is hard, in the process of deformation of the soft phase ferrite dynamic reversion to softening, hard phase austenite dynamic recrystallization softening, and softening of ferrite before austenite, strain is mainly concentrated in the ferrite phase, so that between the two phases there is a deformation of the incongruity, so it is easy to produce micro-cracking in the phase boundary, and ultimately lead to tee forming process branch easy Cracking (tee branch at the largest deformation).
Related research shows that: duplex stainless steel in the compression deformation process, with the compression process, the whole process can be divided into three stages: at the beginning of the stage, the material appears obvious work hardening, in the process accompanied by dynamic reversion softening of ferrite and dynamic recrystallization softening of austenite, but the hardening effect is greater than the softening effect, still shows the work hardening effect, the rheological stress value increases rapidly; in reaching the stress After the peak, the softening effect is greater than the hardening effect, as shown in the rheological stress value decreases, when the rheological stress value tends to stabilize, the hardening effect and softening effect is basically equal, the material enters the steady-state rheological stress stage, at this time, the dynamic recrystallization of austenite occurs. The longer the steady-state zone, the lower the rheological stress, the better the thermoplasticity of the material. Related research also shows that: deformation resistance decreases with increasing temperature, the higher the temperature, the less deformation into the steady-state rheology, the better the thermoplasticity of the material; the less the amount of γ-phase of the material, the higher the temperature of the two phases of the deformation of the incoordination of the buffer, so it is not easy to crack.
In summary, and repeatedly verified by our company, the best forming temperature range of duplex stainless steel tee is 1100 ℃ ~ 1250 ℃. However, in the actual tee forming process, the tee thickness is limited, and the time above 1100℃ after the furnace is very short, so when pressing and forming, the number of pressing passes and the amount of deformation of each pass should be controlled so that the rheological stress is at a lower level, thus preventing the pipe fitting from cracking.

When pressing and forming duplex steel tee, in order to avoid forming cracking, the following matters should be noted:

  • 1) the press power should be matched;
  • 2) the pressing process should be uniform and slow;
  • 3) control the number of passes of pressing.

3. Heat treatment of duplex steel tee after forming

In order to obtain good corrosion resistance and hot workability of duplex stainless steel, the ratio of ferrite and austenite phases in the steel should be close to 1:1. Control the phase ratio of the two phases in duplex stainless steel is generally used to control the chemical composition of duplex stainless steel and select the appropriate heat treatment specification to achieve.

20220220111019 97544 - Discussion on forming and heat treatment process of duplex steel tee

As the actual tee making process is impossible to regulate the chemical composition of the material, however, we can determine the dual-phase ratio of the raw material in equilibrium by calculating the Cr equivalent and Ni equivalent of the raw material, and then according to the Schaeffler organization diagram, the dual-phase ratio of the raw material to develop the heat treatment process parameters that are compatible with it, so as to finally obtain the dual-phase ratio we need.
The study shows that.

  • (1) The double comparison of the same material is different for different solid solution temperatures, with the austenite phase gradually decreasing and the ferrite phase gradually increasing as the solid solution temperature increases.
  • (2) The equilibrium phase ratio of duplex stainless steel can be determined by calculation, the raw materials are different, due to differences in their chemical composition, so their equilibrium phase ratio is also different, to get the same phase ratio of the organization, can be adjusted by heat treatment temperature.

Therefore, for our pipe fittings manufacturing, the correct heat treatment specification should be determined by the following steps.
Firstly, calculate the double comparison of raw materials in equilibrium according to the furnace number of raw materials, and roughly determine the heat treatment solid solution temperature range according to this result.
Secondly, take specimens by furnace number to simulate thermoforming, take specimens on thermoformed specimens, conduct precise heat treatment tests according to the calculated temperature range, test the phase ratio, and determine the heat treatment process specification according to the customer’s requirements.
When the product is finally heat-treated, bring along with the furnace specimen or test block to determine whether there is deviation from the process specification through the inspection of the specimen or test block to ensure the final qualification of the product.
Duplex steel tee solution treatment precautions.

  • 1) The control of furnace loading temperature, the furnace is first heated to 1000 ℃, and then loaded, the purpose is to prevent the production of δ phase (harmful phase) due to excessive heating time in the sensitization temperature zone. Workpiece into the furnace temperature shall not be less than 850 ℃, and the maximum power heating.
  • 2) Thermal insulation, when the workpiece temperature reaches the set thermal insulation temperature, and evenly consistent after the start of the insulation state, and start to calculate the insulation time. The holding time is 1.5min/mm (workpiece thickness: the actual maximum wall thickness of the workpiece in the same furnace product shall prevail), and not less than 30min.

The effect of solid solution treatment temperature on the mechanical properties of duplex steel has two main factors: the complete degree of grain recrystallization and the degree of transformation of α and γ phases. With the increase of the solid solution treatment temperature, the complete degree of grain recrystallization and the trend of growth, the organization of the α, γ two-phase transformation that ferrite increased, austenite reduced.
Research shows that the solid solution treatment temperature below 1050 ℃, the strength of the temperature increases and decreases, which is due to the recrystallization of the tissue plays a leading role, and therefore the strength of steel gradually decreases, but its impact toughness increased significantly; when the solid solution treatment temperature is higher than 1050 ℃, the strength increases with the increase in temperature, the transformation of the two-phase organization plays a leading role, and therefore the strength of steel gradually increases, its impact toughness decreasing trend. When the solution treatment temperature is greater than 1100 ℃, its strength begins to decline, but its impact toughness change is not obvious, when the solution treatment temperature reaches 1150 ℃, its tensile strength has been lower than 800 MPa, mainly due to the solution treatment temperature is too high, resulting in the organization of significant recrystallization growth, so its strength is reduced; in addition, the higher the solution treatment temperature, resulting in increased energy consumption.

Commonly used duplex steel materials recommended the following temperature and test hardness.

Material Temperature () Brinell hardness (HB)
S31803 1050-1100 ≤290
S32205 1050-1100 ≤290
S32750 1050-1100 ≤310

4. Organizational inspection of duplex steel tee after solid solution treatment

After solid solution treatment of duplex stainless steel tee requires control of the phase ratio, the most appropriate ratio is about half of the ferrite phase and austenite phase, the number of a phase can not exceed 65% at most, so as to ensure the best overall performance. If the proportion of the two phases is out of balance, for example, the number of ferrite phase is too much, it is easy to form a single-phase ferrite in the welding HAZ, in some media sensitive to stress corrosion rupture.
Therefore, the solid solution treatment requires the determination of the phase ratio of each tee, a phase ratio of more than 65% should be judged as unqualified.

5. Concluding remarks.

The significant advantages of duplex steel materials have led to their massive application in the market. For dual-phase steel pipe parts, due to the existence of two key processes of forming and heat treatment, whether appropriate will directly lead to the final quality of the product. According to our past experience of supply, qualified products are not only perfect in surface quality, but more important in internal quality, mechanical properties and reasonable organization ratio. The process of forming and heat treatment of duplex steel tee still needs to be further explored and researched, and with the follow-up of equipment and tooling, there is still room to enhance and improve the specific implementation of the process parameters.

Source: Network Arrangement – China Duplex Steel Tee Manufacturer: 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 [email protected]

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