Precipitation hardening stainless steel heat treatment
Precipitation hardening stainless steel is relatively late in the development of stainless steel species after testing, summary and innovation in human practice. The early appearance of stainless steel, ferritic stainless steel, austenitic stainless steel has better corrosion resistance, but can not adjust the mechanical properties through heat treatment method, limiting its role. The martensitic stainless steel can use heat treatment methods to adjust the mechanical properties in a larger range, but the corrosion resistance is poor.
Features.
It has a lower amount of C (generally ≤ 0.09%), a higher amount of Cr (generally ≥ 14% above), plus Mo, Cu and other elements, which makes it has a higher corrosion resistance, and even with austenitic stainless steel equivalent. Through solid solution and aging treatment, the precipitation hardening phase can be precipitated on the martensitic matrix of the organization, thus having higher strength, and can be adjusted according to the adjustment of the aging temperature, within a certain range of strength, plastic, toughness. In addition, the first solution, and then in accordance with the precipitation phase precipitation strengthening of the heat treatment method, after solution treatment, the lower hardness of the case of processing basic molding, and then by the aging strengthened, reducing processing costs, superior to martensitic steel.
Classification: precipitation hardening stainless steel: martensitic, semi-austenitic, austenitic, austenitic + ferritic
①Martensitic precipitation hardening stainless steel and its heat treatment
Martensitic precipitation-hardening stainless steel is characterized by an austenitic to martensitic transformation starting at a temperature Ms above room temperature. After austenitization by heating and cooling at a faster rate, a slate-like martensitic matrix is obtained, which is reinforced by the precipitation of fine masses of Cu from the slate-like martensitic matrix after aging.
Example: 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°C; Mz point is about 30°C.
Solid solution treatment.
Heating temperature of 1020-1060 ℃, water-cooled or oil-cooled after insulation, the organization is slate-like martensite, hardness about 320HB. Heating temperature should not be too high, if it is greater than 1100 ℃, will increase the amount of ferrite in the organization, Ms point down, residual austenite increase, hardness decreased, heat treatment effect is not good.
Aging treatment.
Depending on the aging temperature, the dispersion and particle size of precipitate precipitates are different, and have different mechanical properties.
After aging performance of different aging temperature
Semi-austenitic stainless steel heat treatment
This steel’s Ms point is generally slightly below room temperature, so the solution treatment cooled to room temperature, austenitic organization, strength is very low, in order to improve the strength of the matrix, hardness, need to be heated again to 750-950 ℃, holding, this stage, austenite will precipitate carbide, austenite stability is reduced, Ms point to room temperature above, and then cooled, martensite organization. Some can also add a cold treatment (subzero treatment), after which the steel is then aged so that the steel is finally reinforced with precipitated precipitates on the martensitic matrix.
Example: PH17-7 composition (%): C ≤ 0.09, Cu ≤ 0.5, Ni:6.5-7.5, Cr:16-18, Al:0.75-1.5;
Solid solution + adjustment + aging treatment
– Solid solution heating temperature of 1040 ℃, heating and holding water-cooled or oil-cooled austenite, hardness of 150HB or so;
– Adjust the processing temperature of 760 ℃, after holding air-cooled, so that the austenite alloy carbide precipitation, reduce the austenite stability, improve the Ms point to 50-90 ℃ or so, after cooling to obtain slat martensite, this time the hardness can reach 290HB or so;
– Then by 560 ℃ aging, Al and compounds precipitation precipitation precipitation, steel strengthening, hardness up to 340HB or so.
Solid solution + adjustment + cold treatment + ageing
– The solid solution treatment was heated to 1040°C, water cooled and austenitic tissue was obtained;
– Adjust the treatment temperature to 955°C, increase the Ms point, and obtain slate-martensite after cooling;
– Cold treatment at -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 precipitates out, and the hardness can reach 336HB after strengthening treatment.
Solid solution + cold deformation + aging
– Solid solution treatment temperature of 1040°C, water cooling, to obtain austenite tissue;
– Cold deformation, the use of cold working deformation strengthening principle, so that the austenite at the Md point into martensite, the cold working deformation amount to be more than 30-50%;
– Ageing treatment: heat ageing at 490℃, so that Al precipitates and hardens.
– Some reports show that the solid solution austenite deformed by 57% cold rolling, hardness up to 430HB, σb up to 1372 N/mm2, and then by 490 ℃ aging, hardness up to 485HB, σb up to 1850 N/mm2.
It can be seen, precipitation hardening martensitic stainless steel after the correct treatment, mechanical properties can be achieved martensitic stainless steel performance, while the corrosion resistance is equivalent to austenitic stainless steel. It should be noted here that, although martensitic stainless steel and precipitation-hardened stainless steel can be strengthened by heat treatment, but the strengthening mechanism is different. Due to the characteristics of precipitation-hardening stainless steel, so that it is valued and widely used.
