Stainless Steel Heat Treatment
Stainless steel is characterized by its composition, which is made up of a large number of alloying elements with Cr as the main component. This is the fundamental requirement for stainless steel to be resistant to corrosion.
To fully utilize the alloying elements and achieve optimal mechanical and corrosion resistance, heat treatment methods must also be employed.
1 . Heat Treatment of Ferritic Stainless Steel
Ferritic stainless steel is typically characterized by a stable single ferrite structure and does not undergo phase change upon heating and cooling.
As a result, heat treatment cannot be used to adjust its mechanical properties. The main objective is to reduce brittleness and increase resistance to intergranular corrosion.
- Brittleness of the σ phase: Ferritic stainless steel has a tendency to form the σ phase, which is a metallic compound rich in Cr, hard and brittle. This formation is facilitated by the presence of elements such as Cr, Si, Mn and Mo and by heating the steel to temperatures between 540 and 815°C. However, the formation of the σ phase is reversible and reheating above its formation temperature will redissolve it into a solid solution.
- Brittleness at 475°C: When ferritic stainless steel is heated for a prolonged period within the range of 400-500°C, it may exhibit increased strength, decreased toughness and increased brittleness, especially at 475°C. This occurs because the Cr atoms in ferrite will reorganize and form Cr-rich regions that cause lattice distortion and generate internal stress, resulting in increased hardness and brittleness. The formation of these Cr-rich regions also reduces the corrosion resistance of the steel. Reheating to a temperature above 700°C will eliminate distortion and internal stress, and brittleness at 475°C will disappear.
- High-temperature brittleness: Rapid cooling after heating ferritic stainless steel above 925°C can cause precipitation of compounds such as Cr, C and N in grains and grain boundaries, leading to increased brittleness and intergranular corrosion. This can be remedied by heating the steel to temperatures between 750 and 850°C and then cooling rapidly.
Heat treatment process:
① Annealing
To eliminate σ phase, 475°C brittleness and high temperature brittleness, annealing treatment can be applied.
The process involves heating to 780~830°C, followed by air cooling or oven cooling.
For ultrapure ferritic stainless steel with low C content (C≤0.01%) and strictly controlled levels of Si, Mn, S and P, the annealing temperature can be elevated.
② Stress relief treatment
After welding or cold working, parts may contain residual stresses.
In cases where annealing is not suitable, stress relief treatment can be carried out by heating the parts to a temperature of 230~370°C, maintaining the temperature and then cooling in air. This can help eliminate some internal stress and improve plasticity.
two . Austenitic Stainless Steel Heat Treatment
The presence of Cr, Ni and other alloying elements in austenitic stainless steel reduces the Ms point below room temperature (-30 to -70°C).
This stability of the austenitic structure means that no phase change occurs during heating and cooling above room temperature.
The main objective of heat treating austenitic stainless steel is, therefore, not to change the mechanical properties, but rather to increase corrosion resistance.
Solution Treatment of Austenitic Stainless Steel
Effects:
① Precipitation and Dissolution of Alloy Carbides in Steel
Carbon (C) is one of the alloying elements present in steel. Although it has a slight strengthening effect, it is detrimental to corrosion resistance, especially when it forms carbides with chromium (Cr).
To minimize the existence of C and Cr carbides, the solubility of C in austenite is manipulated by heating and cooling.
The solubility of C in austenite is high at high temperatures (0.34% at 1200°C) and low at low temperatures (0.02% at 600°C, and even lower at room temperature).
The steel is heated to a high temperature to dissolve the C-Cr compound and cooled quickly to prevent precipitation.
This helps to improve the corrosion resistance of steel, especially its resistance to intergranular corrosion.
② Sigma Phase (σ)
Long-term heating in the range of 500-900°C or the addition of elements such as titanium, niobium and molybdenum can result in precipitation of the σ phase in austenitic steel.
This increases the fragility of the steel and reduces its resistance to corrosion.
The σ phase can be eliminated by dissolving it at a temperature higher than its precipitation temperature and cooling it rapidly to prevent reprecipitation.
Process:
According to the GB1200 standard, the recommended heating temperature range is 1000-1150°C, generally 1020-1080°C.
The heating temperature can be adjusted within the allowable range based on the specific composition of the grade, castings or forgings. The cooling method must be rapid to avoid carbide precipitation.
In China and some other national standards, “rapid cooling” is indicated after solid solution.
The scale of “fast” can be determined based on the following criteria:
- For C content ≥ 0.08% or Cr content > 22% and amount of Ni, the steel must be water cooled.
- For C content < 0.08% and size > 3 mm, the steel must be air cooled.
- For C content <0.08% and size ≤0.5 mm, the steel can be air cooled.
Austenitic Stainless Steel Stabilization Heat Treatment
Stabilization heat treatment is a process limited to specific grades of austenitic stainless steels, such as 1Cr18Ni9Ti and 0Cr18Ni11Nb, which contain Ti or Nb stabilizing elements.
Effects:
As discussed previously, precipitation of Cr23C6-type compounds due to the combination of Cr and C at grain boundaries can lead to a decline in the corrosion resistance of austenitic stainless steel.
To avoid this, Ti and Nb are added to the steel to create conditions where C preferentially combines with Ti and Nb rather than Cr.
This helps retain the Cr in the austenite and ensures the steel's corrosion resistance. Stabilization heat treatment combines Ti, Nb and C to stabilize Cr in austenite.
Process:
Heating temperature: The heating temperature should be higher than the dissolution temperature of Cr23C6 (400-825 ℃) and slightly lower or higher than the initial dissolution temperature of TiC or NbC (for example, the dissolution temperature range of TiC is 750-1120 ℃).
The stabilization heating temperature is generally set at 850-930°C, which completely dissolves Cr23C6 and allows Ti or Nb to combine with C, while retaining Cr in the austenite.
Cooling method: Air cooling is typically used, but water cooling or furnace cooling can also be used depending on the specific conditions of the parts.
The cooling rate has minimal impact on the stabilization effect.
Our experimental research has shown that cooling rates of 0.9°C/min and 15.6°C/min from a stabilized temperature of 900°C to 200°C result in similar metallographic structure, hardness and resistance to intergranular corrosion .
Austenitic Stainless Steel Stress Relief Treatment
Purpose:
Austenitic stainless steel parts inevitably experience stress during cold working processes such as processing and welding.
This stress can have negative effects, such as impacting dimensional stability and causing stress corrosion cracking in media such as Cl-, H2S, NaOH, etc.
This type of damage is local and sudden and can be harmful. To minimize stress on these parts, stress relief methods can be used.
Process:
Solution treatment and stabilization treatment can help eliminate stress if conditions permit. However, these methods may not always be feasible, such as in the case of ring pipe fittings, finished parts with limited margins, and parts with complex shapes that are easily deformable.
In these cases, heating the parts to a temperature below 450°C can help reduce stress.
If the workpiece is used in a strong stress corrosion environment and stress needs to be completely eliminated, consideration should be given to selecting materials such as ultra-low carbon austenitic stainless steel with stabilizing elements.
3. Heat treatment of martensitic stainless steel
The most distinctive feature of martensitic stainless steel compared with ferritic stainless steel, austenitic stainless steel and duplex stainless steel is its ability to adjust its mechanical properties over a wide range through heat treatment methods to meet the diverse needs of different applications.
Furthermore, the corrosion resistance of martensitic stainless steel can be affected differently by the different heat treatment methods used.
① The structure of martensitic stainless steel after quenching
Depending on the chemical composition
- 0Cr13, 1Cr13, 1Cr17Ni2 are martensite + a small amount of ferrite;
- 2Cr13, 3Cr13, 2Cr17Ni2 are basically martensitic structure;
- 4Cr13, 9Cr18 are alloy carbides in martensite matrix;
- 0Cr13Ni4Mo and 0Cr13Ni6Mo retained austenite in the martensite matrix.
② Corrosion resistance and heat treatment of martensitic stainless steel
The heat treatment of martensitic stainless steel not only changes its mechanical properties, but also affects its corrosion resistance in several ways.
For example, low temperature tempering after quenching results in high corrosion resistance, while medium temperature tempering (400-550°C) results in low corrosion resistance.
On the other hand, high temperature tempering (600-750°C) leads to better corrosion resistance.
③ The heat treatment process method and function of martensitic stainless steel
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Different annealing methods can be used depending on the desired result:
- Low-temperature annealing (sometimes called incomplete annealing) can be used if the goal is to reduce hardness, ease processing, and relieve stress. The heating temperature is generally between 740-780°C, and the hardness can be maintained at 180-230HB after air cooling or furnace cooling.
- Complete annealing is used if the objective is to improve the structure of the forging or casting, decrease hardness and ensure low performance. This method typically involves heating the material to 870-900°C and cooling it in a furnace, or cooling it below 600°C at a rate of 40°C/h or less. The hardness after this process can be between 150-180HB.
- Isothermal annealing is an alternative to complete annealing and can achieve the same objective. The material is heated to 870-900°C, held at that temperature, then cooled to 700-740°C (see transformation curve), held for longer (see transformation curve), and finally cooled below 550°C . The hardness after this process can also be between 150-180HB.
This isothermal annealing process is also effective in improving poor structure after forging, as well as improving mechanical properties after quenching and tempering, particularly impact resistance.
Tempera
The main purpose of quenching martensitic stainless steel is to increase its strength.
The process involves heating the steel to a temperature above the critical point, maintaining the heat to ensure that the carbides completely dissolve into austenite, and then cooling at an appropriate rate to obtain a martensite structure.
Heating temperature selection: The basic principle is to form austenite and dissolve alloy carbides homogeneously in the austenite.
To avoid coarser austenite grains or the presence of ferrite or austenite retained in the structure after quenching, the heating temperature should not be too low or too high.
The temperature range for quenching martensitic stainless steel varies widely, but in our experience it is typically between 980-1020°C.
However, for special steel grades, specific composition control or particular requirements, the heating temperature may need to be adjusted, but the heating principle must not be violated.
Cooling method: Due to the composition of martensitic stainless steel, austenite is more stable, the C curve shifts to the right, and the critical cooling rate is lower.
Therefore, martensitic steel can be hardened using oil cooling or air cooling.
However, for parts that require a large hardening depth and high mechanical properties, especially high impact toughness, oil cooling is recommended.
Temperament
After quenching, martensitic stainless steel is obtained with high hardness, brittleness and internal stresses, which must be tempered to improve its mechanical properties.
Martensitic stainless steel is typically hardened at two different temperatures:
- Tempering between 180 and 320°C results in a tempered martensite structure that maintains high hardness and strength, but has low plasticity and toughness, with good corrosion resistance. This structure is ideal for applications such as cutting tools, bearings and wear parts.
- Tempering between 600 and 750°C results in a quenched sorbite structure that has a good balance between strength, hardness, plasticity and toughness, with good corrosion resistance. Depending on the desired mechanical properties, tempering may be used at the lower or upper end of this temperature range.
Tempering at a temperature between 400 and 600°C is generally not recommended as it can cause precipitation of highly dispersed carbides from martensite, resulting in temper brittleness and reduced corrosion resistance.
However, some springs, such as 3Cr13 and 4Cr13 steel springs, can be tempered at this temperature, resulting in an HRC of 40 to 45 and good elasticity.
The cooling method after tempering is generally air cooling, but for steel grades prone to temper brittleness, such as 1Cr17Ni2, 2Cr13 and 0Cr13Ni4Mo, oil cooling is recommended after tempering.
4 . Heat treatment of ferritic-austenitic duplex stainless steel
Duplex stainless steel is a recent addition to the stainless steel family and has gained widespread recognition and appreciation for its unique characteristics.
Its high chromium content, low nickel composition, and added molybdenum and nitrogen make it stronger and more flexible than austenitic and ferritic stainless steels while providing equivalent corrosion resistance.
It also has superior resistance to pitting, crevice and stress corrosion in chloride and seawater environments.
The effects of heat treatment for duplex stainless steel are as follows:
① Eliminate Secondary Austenite: At higher temperatures, such as during casting or forging, the amount of ferrite increases.
At temperatures above 1300°C, it can transform into single-phase ferrite, which is unstable at high temperatures. Aging at lower temperatures can result in the precipitation of austenite, known as secondary austenite.
However, the amount of chromium and nitrogen in this austenite is lower than in normal austenite, making it a potential source of corrosion, so it must be removed through heat treatment.
② Eliminate Cr23C6 carbide: Duplex steel can precipitate Cr23C6 at temperatures below 950°C, causing increased brittleness and reduced corrosion resistance. This must be eliminated.
③ Eliminate Cr2N, CrN Nitrides: Due to the presence of nitrogen in steel, nitrides can form with chromium, which can negatively impact both mechanical properties and corrosion resistance, and must be eliminated.
④ Eliminate the intermetallic phase: The composition of two-phase steel can result in the formation of intermetallic phases, such as σ phase and γ phase, which reduce corrosion resistance and increase brittleness, so they must be eliminated.
The heat treatment process is similar to that of austenitic steel and involves solid solution treatment with a heating temperature of 980~1100°C followed by rapid cooling. Water cooling is typically used.
5 . Heat Treatment of Precipitation Hardened Stainless Steel
Precipitation hardened stainless steel is a relatively recent development and is a type of stainless steel that has been tried, tested and improved through human practice.
Previous stainless steels such as ferritic and austenitic stainless steels have good corrosion resistance, but their mechanical properties cannot be adjusted through heat treatment methods, which restricts their usefulness.
Martensitic stainless steel can be heat treated to further adjust its mechanical properties, but its corrosion resistance is low.
Characteristics:
Precipitation-hardened stainless steel has a low carbon content (generally ≤0.09%) and high chromium content (generally ≥14% or higher), along with elements such as Mo and Cu, making it have corrosion resistance equivalent to of austenitic stainless steel.
Through a solid solution and aging treatment, a structure with precipitation hardening phases precipitated in the martensitic matrix can be obtained, resulting in greater strength.
The strength, plasticity and toughness can be adjusted within a certain range by adjusting the aging temperature.
Furthermore, the method of solid solution heat treatment followed by precipitation strengthening in the precipitation phase allows the processing of basic shapes with low hardness after solid solution treatment.
By strengthening again through aging, processing costs are reduced and surpass martensitic steels.
Classification:
① Stainless steel with martensitic precipitation hardening and its heat treatment
Precipitation-hardened martensitic stainless steel is characterized by an austenitic to martensitic transformation beginning above room temperature (Ms).
By heating the steel to its austenitizing temperature and cooling it rapidly, a slate-like martensitic matrix is obtained.
After aging, the fine mass of copper precipitates from the martensitic matrix, strengthening the steel.
A typical grade in the GB1220 standard is 0Cr17Ni4Cu4Nb (PH17-4), with the following composition: C≤0.07, Ni: 3-5, Cr: 15.5-17.5, Cu: 3-5, Nb: 0 .15-.45. The Ms point is approximately 120°C and the Mz point is approximately 30°C.
Solid solution treatment:
When heated to 1020-1060°C and quickly cooled with water or oil, the steel structure becomes lath martensite, with a hardness of around 320HB.
The heating temperature should not exceed 1100°C, as this may result in an increase in ferrite in the structure, a decrease in the Ms point, an increase in retained austenite, a decrease in hardness and poor heat treatment effects.
Aging Treatment:
The dispersion and particle size of the precipitates depend on the aging temperature and result in different mechanical properties.
According to the GB1220 standard, the properties after aging at different temperatures are as follows:
② Heat treatment of semi-austenitic stainless steel
The Ms point of semi-austenitic stainless steel is generally slightly below room temperature, resulting in an austenite structure with low strength after solution treatment and cooling to room temperature.
To improve the strength and hardness of the matrix, the steel needs to be reheated to 750-950°C for insulation.
At this stage, carbides will precipitate in the austenite, reducing its stability and increasing the Ms point above room temperature.
After cooling, a martensitic structure is obtained. Cold treatment (subzero treatment), followed by aging, can also be added to produce a steel reinforced with precipitates in the martensitic matrix.
A recommended grade in the GB1220 standard is 0Cr17Ni7Al (PH17-7) with the following composition: C≤0.09, Cu≤0.5, Ni: 6.5-7.5, Cr: 16-18, Al: 0, 75-1.5.
Solution + Adjustment + Aging Treatment:
The temperature of the solid solution is 1040°C and the steel is cooled with water or oil to obtain an austenite structure with a hardness of around 150HB.
The setting temperature is 760°C, and the steel is cooled in air to precipitate alloy carbides in austenite, reduce its stability, increase the Ms point to 50-90°C, and obtain lath martensite after cooling. The hardness can reach 290HB.
After aging at 560°C, Al and its compounds precipitate, strengthening the steel and increasing its hardness to 340HB.
Solid Solution + Adjustment + Cold Treatment + Aging:
The temperature of the solid solution is 1040°C, and water cooling is used to obtain an austenite structure.
The setting temperature is 955°C to increase the Ms point and obtain lathed martensite after cooling.
Cold treating at -73°C for 8 hours reduces the austenite retained in the structure to obtain maximum martensite.
Classification and Main Characteristics of Stainless Steel
There are numerous ways to classify stainless steel, including based on chemical composition, functional properties, metallographic structure and heat treatment characteristics.
However, for practical reasons, it is more useful to categorize it based on its metallographic structure and heat treatment characteristics.
1 . Ferritic stainless steel
The main alloying element of stainless steel is chromium, and a small amount of stable ferrite elements such as aluminum and molybdenum can be added. The resulting structure is ferrite.
This type of stainless steel has low strength and cannot be improved through heat treatment.
Instead, it has some plasticity but also great fragility. It has good corrosion resistance in oxidizing media (such as nitric acid), but poor corrosion resistance in reducing media.
two . Austenitic stainless steel
Contains a high concentration of chromium, generally greater than 18%, and approximately 8% nickel.
Some use manganese to replace nickel to further increase corrosion resistance, and some add elements such as molybdenum, copper, silicon, titanium or niobium.
There is no phase change during heating and cooling, so heat treatment methods cannot be used to increase its strength.
However, it has the advantages of low strength, high plasticity and high toughness. It is highly resistant to oxidizing media and presents good resistance to intergranular corrosion after the addition of titanium and niobium.
3 . Martensitic stainless steel
Martensitic stainless steel mainly contains 12-18% Cr, with the amount of carbon adjustable according to needs, typically 0.1-0.4%.
For tools, the carbon content can reach 0.8-1.0%, and some are improved by adding elements such as Mo, V and Nb to increase stability and tempering resistance.
Heating to high temperatures and cooling at a certain rate results in a structure that is primarily martensitic, but may also contain small amounts of ferrite, retained austenite, or alloying carbides, depending on the carbon content and alloying elements.
The structure and performance can be adjusted by controlling the heating and cooling process, but the corrosion resistance is not as good as that of austenitic, ferritic and duplex stainless steels.
Martensitic stainless steel is resistant to organic acids, but has low resistance to media such as sulfuric and hydrochloric acids.
4 . Austenoferritic steel stainless steel
Generally, the Cr content is 17 to 30% and the Ni content is 3 to 13%.
Furthermore, alloying elements such as Mo, Cu, Nb, N and W are added, and the C content is kept very low.
Depending on the proportion of alloying elements, some are ferrite while others are mainly austenite, constituting two duplex stainless steels that exist simultaneously.
As it contains ferrite and reinforcing elements, after heat treatment its resistance is slightly higher than that of austenitic stainless steel and its plasticity and toughness are better.
Performance cannot be adjusted through heat treatment.
It has high resistance to corrosion, especially in media containing Cl and seawater, and has good resistance to pitting corrosion, crevice corrosion and stress corrosion cracking.
5 . Precipitation hardened stainless steel
The composition of this type of stainless steel is characterized by the presence of elements such as C, Cr, Ni and other elements, including Cu, Al and Ti, which can cause precipitation.
The mechanical properties can be adjusted through heat treatment, but its strengthening mechanism differs from that of martensitic stainless steel.
Due to its dependence on precipitation-based strengthening, the carbon content can be kept very low, resulting in better corrosion resistance than martensitic stainless steel and equivalent to austenitic Cr-Ni stainless steel.
