Precipitation hardened stainless steels
An Achilles heel of austenitic stainless steels is the susceptibility to stress corrosion cracking (SCC). However, when the nickel concentration exceeds about 20% considerable improvement in the resistance to stress corrosion is observed (Fig 1). Nickel-rich austenitic stainless steels (NiASS), therefore, deserve to be treated as an own family. In fact, for nickel concentrations beyond about 30% the resistance to stress corrosion is comparable to that of duplex and ferritic stainless steels.
Any object hindering dislocation motion raises the strength of metallic materials. Strengthening by particles (precipitates) was observed for the first time in 1911 in aluminium alloys by Wilm. The effect is so important that one family of stainless steels, so-called precipitation hardening stainless steels (PHSS), derives its name from this phenomenon. However, the first systematic use of precipitate hardening took place during World War II when US Steel launched “Stainless W” (UNS S 17600).
An advantage of PHSS over martensitic stainless steels is the higher corrosion resistance. While a significant amount of chromium in martensitic stainless steels is tied to carbides and, therefore, rendered inefficient for corrosion resistance, PHSS rely upon the formation of intermetallic precipitates devoid of chromium. PHSS are quite formable owing to the relatively low carbon concentration in the martensite. This circumstance is used in many applications where severe plastic deformation is required during manufacturing of components. The precipitation strengthening is then performed by a final tempering treatment.
|Alloy||UNS No.||Composition, %|
|PH 13-8 Mo||S13800||0.05||0.10||0.10||12.8||8.0||2.3||Al=1.1|
PHSS are typically based on iron, chromium and nickel with one or more of the following elements copper, aluminium, titanium, niobium and molybdenum. A list of a selection of PHSS is shown in Table 1. Common precipitates are face centered cubic Cu, Laves phase, hexagonal Ni3Ti and ordered g’ (Ni3(Ti, Al)). Owing to the low solubility of copper at the tempering temperature, clusters of elemental copper are often formed at an early stage of tempering, thereby facilitating the nucleation of other phases. This phenomenon can be observed in Fig 1 after 5 min of ageing at 475°C of Sandvik Nanoflex. The image, in which each dot is an individual atom, was produced using atom probe field ion microscopy. The case shown in Fig 1 illustrates the embryonic stage of tempering where nickel-rich precipitates (magenta) start to form at copper clusters (yellow) in a martensitic matrix.
It deserves to be mentioned that the martensite formed in Sandvik Nanoflex is rather unusual as it forms during an isothermal heat treatment at cryogenic temperatures. In previous columns we have treated cooling-induced and strain-induced martensites. Isothermal martensite is a third type of martensite that is quite rare and seldom used in practice. This enables the product to be formed in a soft state and subsequently hardened by a heat treatment.
The choice of tempering treatment (viz. time and temperature) depends upon the required properties of the final component and is the result of a compromise. For instance, maximum hardness gives optimum strength but may result in insufficient toughness. In some applications, therefore, it may be wise to interrupt the tempering treatment before peak hardening has taken place.
PHSS are subdivided into martensitic, semi-martensitic and fully austenitic alloys. The martensitic alloys have an Mf temperature so high that the transformation from austenite to martensite is completed at room temperature (e.g. PH13-8Mo and 17-4PH). Semi-martensitic alloys are almost fully austenitic at room temperature but, owing to the metastability of austenite, the final transformation to martensite occurs during plastic deformation (e.g. 17-7PH and PH15-7Mo). An advantage is that fabrication and manufacturing can take place in a rather soft condition after which the final strength is attained by tempering with little distortion. The subgroup of fully austenitic alloys is quite small but serves the purpose in applications where high strength in combination with non-ferromagnetism is desired.
Martensitic PHSS are required in many aircraft applications including valve parts, fittings, landing gear parts, shafts, pins, and lock-washers. The semi-martensitic PHSS are attractive when severe plastic deformation is required to obtain the shape of the final component. Also these steels are used in very specific aerospace applications where a combination of strength, corrosion resistance and formability is required.
There are a variety of applications in the medical industry where similar properties are required. PHSS have found applications in surgical instruments such as surgical needles (Fig 2) and dental reamers (Fig 3). Similar types of PHSS are also used as parts in the shaving head of electric shavers. Sandvik Nanoflex has been found suitable for this purpose an example of which is shown in Fig 4. The cap, which is provided with slits through which the hair can enter into the shaving knife, has for many years been manufactured from this particular alloy.
This article was first published in Stainless Steel World Magazine in December 2017.