Martensitic and precipitation hardening stainless steels are heat treatable and can therefore provide hardness and strength in a wide range. Allowing to workability they are supplied in solution annealed condition. The downstream manufacturer performs final heat treatment to meet the mechanical properties required.
Martensitic grades are basically Fe-Cr alloys with a higher carbon content than ferritics which enables them to harden on cooling in air, oil or water. Depending on grade and intended use, ductility is improved by tempering.
Typical applications for martensitic grades:
- cutting utensils
- surgical and dental instruments
- fasteners, springs and ball bearings
- press plates
- steam and gas stainless steel tubes
Precipitation hardening grades have higher alloying contents than martensitic grades. They contain nickel, and in order to achieve hardening by aging additions of copper, aluminium, titanium, niobium and molybdenum. Depending on chemical composition their microstructure after final heat treatment is austenitic, semi-austenitic or martensitic.
Typical applications for precipitation hardening grades:
- retaining rings, spring holders, springs
- chains, valves and gears
- aircraft parts
- pressure vessels and stainless steel tubes
Grade properties
High strength and hardness distinguish martensitic stainless steels from the other stainless steel families. After austenitizing cooling is performed in air, water or oil, depending on steel grade. If the intended application requires a high level of hardness (e.g. knives, HRC55), only stress relief annealing will be performed. Normally martensitic stainless steels are tempered in order to acquire useful mechanical properties, i.e. a certain level of toughness (A5 ≥ 15 %).
Nickel-martensitic steels are superior to traditional martensitic grades regarding strength in combination with toughness. Their microstructure contains stable austenite after hardening and tempering which accounts for good toughness without drawbacks concerning corrosion resistance.
Precipitation hardening stainless steels provide remarkable levels of high strength and hardness in a very wide range. With the exception of the martensitic alloys (e.g. 1.4542) cold formability is satisfactory.
Weldability
Traditional martensitic steels with a carbon content > 0.20 % are difficult to weld; assistance is advised. The hardenable high-carbon grades are not suitable for welding.
Low-carbon nickel-martensitic grades have relatively good weldability.
Welding of precipitation hardened grades is possible, but depending on grade some limitations might have to be regarded.
Corrosion resistance
Corrosion resistance of martensitic stainless steels may vary considerably depending on chemical composition (C, Cr, Mo), surface finish and especially heat treatment. Smooth polished surfaces experience higher resistance than rougher finishes. In terms of heat treatment the hardened condition is more favourable, since the elements promoting corrosion resistance are in solution and therewith effective. Tempering may lead to carbide precipitation which impairs corrosion resistance. This is always the case for traditional martensitic grades, whereas nickel-martensitic grades with max 0.06 % carbon and 3-6 % nickel (e.g. 1.4313 and 1.4418) do not sacrifice corrosion resistance by tempering.
Corrosion resistance of precipitation hardening steels is higher compared with heat treatable martensitic stainless steels ranking between ferritic Cr and austenitic CrNi steels.
