The precipitation hardening stainless steels (PH SS)

Precipitation hardening stainless steels (PH SS) are basically used in wide range of applications, including the petroleum, aerospace, nuclear industries, and so on. Indeed, the good combination of high corrosion resistance, high strength and hardness (surface-based durability) as well as an almost acceptable degree of toughness (bulk-based durability) can be of their noticeable properties. The classes of PH SS are categorized into three families:

• Low carbon martensitic, such as 17-4PH SS (EN1.4542 / AISI 630)
• Semi-austenitic
• Austenitic The

PH SS is a class of corrosion resistant high alloys some of which is able to be heat treated for providing ultimate tensile strengths (Rm) of 850MPa to 1700MPa and yield strengths of 520MPA to more than 1500MPa that can be up to 3 times higher than austenitic stainless steels (ASS) comprising of the grades 304 or 316.

While the aging temperature of 17-4PH, as the most well-known PH SS, is required around 440 to 490ºC, ASS grades necessitate higher scale of aging temperature, around 700°C. By this heat treatment, the corrosion resistance is to some extent decreased while the oxidation resistance remains very acceptable.

17-4 PH martensitic SS is the product with improved machinability, which gathers excellent corrosion resistance beside high strength and hardness, the optimized combination of the behavior of martensitic and austenitic grades. For example, 44 HRC can be achieved by H900 condition.

All these to the properties of PH SSs lie in the heat treatment process. After solution treatment or annealing, an age hardening stage at low temperature scale is applied for obtaining such required properties such as augmented strength. Low temperature age hardening stage has an advantage of no distortion occurrence; however, there would be some superficial discoloration. Over the hardening stage, a minor shrinkage in size takes place that can be around 0.05% for condition H900 and 0.10% for condition H1150.