Precipitation-hardened Stainless Steel's Heat Treatment
Why choose precipitation-hardened stainless steel?
Precipitation-hardened stainless steel is development shortly in human history. Among the stainless steels that appeared earlier, ferritic stainless steel, austenitic stainless steel has better corrosion resistance, but it cannot be adjusted mechanical properties through heat treatment methods. While martensitic stainless steel can be heat-treated to adjust its mechanical properties over a wide range of applications, but it is less resistant to corrosion. The precipitation-hardened stainless steel can attain high mechanical properties and good resistant.
What is precipitation-hardened stainless steel?
Precipitation-hardened stainless steel has a low amount of C (generally ≤ 0.09%), a high amount of Cr (generally ≥ 14% or more), plus Mo, Cu and other elements, which makes it have a high corrosion resistance, even comparable to austenitic stainless steel. By solid solution and aging treatment, it is possible to obtain tissues that precipitate the hardened phase on the martensitic matrix and thus have high strength. The strength, plasticity and toughness can be adjusted within a certain range according to the aging temperature.
How kind of precipitation-hardened stainless steel?
1. Martensitic precipitation hardened stainless steel
2. Semi-austenitic precipitation-hardened stainless steel
3. Austenitic precipitation hardened stainless steel
4. Austenitic + ferritic precipitation hardened stainless steel
Ⅰ.Heat Treatment Of Martensitic Precipitation-hardened Stainless Steel
The characteristics of martensitic precipitation-hardened stainless steel are: austenitic to martensitic transformation of the start temperature Ms above room temperature. The slatted martensite matrix is obtained by heating the austenitizing and cooling at a faster rate, and is strengthened by the precipitation of fine Cu points from the slatted martensite matrix after ageing.
Example: In GB1220, the typical grade is 0Cr17Ni4Cu4Nb (PH17-4) with the following composition (%): C≤0.07, Ni:3~5, Cr:15.5~17.5, Cu:3~5, Nb:0.15~0.45; Ms point about 120°C; Mz point about 30°C.
1.Solid solution treatment:
Heating temperature of 1020-1060 ℃, water cooling or oil cooling after insulation, the tissue is slate-like martensite, hardness of about 320 HB. The heating temperature should not be too high, if more than 1100 ℃, will cause the amount of ferritin in the tissues to increase, Ms point decline, the residual austenite increase, hardness decline, heat treatment effect is not good.
2.Ageing treatment:
Depending on the ageing temperature, the dispersion and particle size of precipitated precipitates are different, and have different mechanical properties.
Ⅱ.Heat Treatment Of Semi-austenitic Stainless Steel
The Ms point of this steel is generally slightly below room temperature, so the solid solution treatment cooling to room temperature, get austenitic tissue, the strength is very low, in order to improve the substrate strength, hardness, need to heat again to 750-950 ℃, insulation, this stage, the austenite will precipitate carbide, austenitic stability is reduced, Ms point to raise above room temperature, and then cooling, get martensite tissue. In some cases, it is possible to add a cold treatment (sub-zero treatment), after which the steel is aged to obtain a reinforced steel with precipitated precipitates on the martensitic matrix.
Example: In GB1220, the recommended grade of precipitated 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;
1.Solid solution + adjustment + aging treatment
- Solid dissolution heating temperature of 1040 ℃, heating and insulation after water cooling or oil cooling to get austenite, hardness of 150 HB or so;
- Adjust the processing temperature of 760 ℃, insulation after air cooling, so that the austenite in the alloy carbide precipitation, reduce austenitic stability, improve Ms point to 50-90 ℃ or so, after cooling to obtain slat martensite, hardness at this time up to 290 HB or so;
- After 560 ℃, Al and compound precipitation precipitation precipitation, steel strengthening, hardness up to 340HB or so.
2.Solid solution + adjustment + cold treatment + ageing
- Austenitic tissue was obtained by solid solution treatment with heating at 1040°C and water cooling;
- Adjust the treatment temperature to 955°C, raise the Ms point, and obtain slatted martensite after cooling;
- Cold treatment -73℃×8h to reduce the residual austenite in the tissue and obtain the maximum martensite;
- The aging treatment temperature is 510-560 ℃, so that Al precipitation, after intensive treatment, the hardness can reach 336 HB
3.Solid solution + cold deformation + aging
- Solid solution treatment at 1040°C, water-cooled, to obtain austenitic tissue;
- Cold deformation, using the cold process deformation strengthening principle, the austenite is converted to martensite at the Md point, the cold process deformation is greater than 30-50%;
- Aging treatment: heating at about 490°C to make Al precipitation hardening.
- It has been reported that the solid solution austenite is deformed by 57% cold rolled, hardness up to 430HB, σb up to 1372 N/mm2, and then 490℃, hardness up to 485HB, σb up to 1850 N/mm2.
It can be seen that the mechanical properties of precipitation-hardened martensitic stainless steel can fully achieve the martensitic stainless steel properties after correct treatment and the corrosion resistance is comparable to austenitic stainless steel. It should be noted here that martensitic stainless steel and precipitation hardened stainless steel both can be strengthened by heat treatment methods, but the strengthening mechanisms are different! This characteristics make P-H stainless steel valued and widely used.
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