Heat treatment of precipitation hardening stainless steel

Precipitation hardening stainless steel is relatively late in development. It is a kind of stainless steel that has been tested, summarized and innovated in human practice. Among the stainless steels that appeared earlier, ferritic stainless steels and austenitic stainless steels have good corrosion resistance, but the mechanical properties cannot be adjusted by heat treatment methods, which limits their effects. The martensitic stainless steel can use heat treatment to adjust the mechanical properties in a larger range, but the corrosion resistance is poor.

Features of precipitation hardening stainless steel:

It has a lower C content (generally ≤0.09%), a higher Cr content (generally ≥14% or more), plus Mo, Cu and other elements, which makes it have higher corrosion resistance, and even the same Austenitic stainless steel is equivalent. Through solid solution and aging treatment, a structure with precipitation hardening phase precipitated on the martensite matrix can be obtained, so it has a higher strength. According to the adjustment of the aging temperature, the strength, plasticity and toughness can be adjusted within a certain range. In addition, the heat treatment method of first solid solution and then precipitation strengthening according to the precipitation phase can be processed after the solution treatment and the hardness is low, and the basic shape can be processed, and then aging strengthened, which reduces the processing cost and is better than martensitic steel.

Classification of precipitation hardening stainless steel:

Martensitic precipitation hardening stainless steel and its heat treatment

The characteristic of martensitic precipitation hardening stainless steel is that the starting temperature Ms of austenite to martensite transformation is above room temperature. After heating and austenitizing and cooling at a faster rate, a lath martensite matrix is obtained. After aging, Cu fine particles are precipitated from the lath martensite matrix and strengthened.
Example: In the GB1220 standard, the typical grade is: 0Cr17Ni4Cu4Nb (PH17-4)
The composition (%) is as follows: C≤0.07, Ni: 3~5, Cr: 15.5~17.5, Cu: 3~5, Nb: 0.15~0.45; Ms point is about 120℃; Mz point is about 30℃.

Solution treatment:

The heating temperature is 1020-1060℃, after heat preservation, water or oil cooling, the structure is lath martensite, the hardness is about 320HB. The heating temperature should not be too high. If it is higher than 1100℃, the amount of ferrite in the structure will increase, the Ms point will decrease, the retained austenite will increase, the hardness will decrease, and the heat treatment effect will not be good.

Aging treatment:

Depending on the aging temperature, the dispersion and particle size of the precipitates are different, and they have different mechanical properties.
The GB1220 standard stipulates the performance after aging at different aging temperatures.

σb(N/mm2) σs(N/mm2) δ(%) Ψ(%) HB
1040 solid solution ≤363
480×4h ≥1310 ≥1180 ≥10 ≥40 ≥375
550×4h ≥1060 ≥1000 ≥12 ≥45 ≥331
580×4h ≥1000 ≥865 ≥13 ≥45 ≥302
620×4h ≥930 ≥725 ≥16 ≥50 ≥277

Semi-austenitic stainless steel heat treatment

The Ms point of this kind of steel is generally slightly lower than room temperature, so after the solution treatment is cooled to room temperature, an austenite structure is obtained, with very low strength. In order to improve the strength and hardness of the matrix, it needs to be heated to 750-950°C again and kept warm. At this stage, carbides will be precipitated in the austenite, the stability of austenite will decrease, and the Ms point will increase above room temperature. When it is cooled again, a martensite structure will be obtained. Some can also add cold treatment (sub-zero treatment), and then aging to make the steel finally obtain a strengthened steel with precipitates on the martensite matrix.
Example: In the GB1220 standard, the recommended grade of precipitation stainless steel is 0Cr17Ni7Al (PH17-7).
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 solid solution heating temperature is 1040℃, and the austenite is obtained by water cooling or oil cooling after heating and holding, and the hardness is about 150HB;
  • Adjust the treatment temperature to 760℃, and air-cool after heat preservation to precipitate alloy carbides in the austenite, reduce the stability of austenite, increase the Ms point to about 50-90℃, and obtain lath martensite after cooling. At this time, the hardness Up to 290HB;
  • After aging at 560℃, Al and its compounds precipitate out, strengthen the steel, and the hardness can reach about 340HB.

Solution + adjustment + cold treatment + aging

  • Solution treatment heating 1040℃, water cooling, to obtain austenite structure;
  • Adjust the treatment temperature to 955℃, increase the Ms point, and obtain lath martensite after cooling;
  • Cold treatment -73℃×8h, reduce the retained austenite in the structure and obtain the maximum martensite;
  • The temperature of the aging treatment is 510-560℃, so that Al can be precipitated. After strengthening treatment, the hardness can reach 336HB.

Solid solution + cold deformation + aging

  • Solution treatment temperature is 1040℃, water cooling, to obtain austenite structure;
  • Cold deformation, using the principle of cold working deformation strengthening to transform austenite into martensite at the Md point, this cold working deformation is greater than 30-50%;
  • Aging treatment: heating and aging at about 490℃ will make Al precipitate and harden;
  • Reports have shown that solid solution austenite is deformed by 57% cold rolling, the hardness reaches 430HB, σb reaches 1372 N/mm2, and after 490℃ aging, the hardness reaches 485HB, σb reaches 1850N/mm2.

It can be seen that after the precipitation hardening martensitic stainless steel is properly treated, the mechanical properties can fully reach the performance of martensitic stainless steel, but the corrosion resistance is equivalent to that of austenitic stainless steel. It should be pointed out here that although both martensitic stainless steel and precipitation hardening stainless steel can be strengthened by heat treatment, the strengthening mechanism is different. Due to the characteristics of precipitation hardening stainless steel, it has been valued and widely used.

Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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.)

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