The most technically sophisticated and complex of the various types of stainless steels are known as Precipitation Hardening (PH) grades. It is different from the “transformation” hardening commonly used in other non-austenitic steels. The PH designation is a reference to a three-step heat treatment technique, also referred to as age hardening. Precipitation hardening achieves an increase of the tensile and yield strength of the steel significantly above values that can be achieved in non-heat treatable austenitic grades, such as type 304/304L. High temperature properties, such as creep, are also enhanced.
The Precipitation Hardening Process
The PH process works by introducing additional elements into the steel, combinations of aluminum, copper or titanium. When this admixture of materials goes through three step heat treatment (Solution, Quench, Age), fine particles of an intermetallic phase, are produced. The presence of these fine particles, called precipitates, impede the ability of the grains of host material to deform under load, thus increasing the strength.
The first step, Solution, involves heating and soaking the materials at a sufficiently high temperature to cause the critical hardening elements to dissolve in the host material. The second step, Quenching, is the rapid cooling of the alloy, resulting in the generation of a matrix in which the hardening elements are “trapped” in the metallic grains. The quenching process happens so rapidly, however, that precipitates do not have time to form in the matrix. In the final step, Aging, the quenched material is re-heated but at a temperature below that initially used for solution. At this temperature, the hardening elements will combine to appear as a finely dispersed distribution of microscopic precipitates. Depending on the application, the steel can be worked and welded at any stage of the PH process.
PH Stainless Steel Grades
Within the PH grouping of stainless steels, various commonly used grades include: martensitic, semi-austenitic, and duplex.
Type 17-4 PH and 15-5 PH are Martensitic grades.
Type 17-7 PH and PH 15-7 Mo are semi-austenitic grades. They are austenitic in the annealed state, and martensitic in the hardened condition.
Various Duplex grades include type 2205 (22Cr5Ni3Mo), and types 2101, 2102, 2202 and 2304, referred to as “Lean Duplex.” Lean grades are formulated with lower nickel and no molybdenum. This contributes to improved weldability compared to other grades. When compared to the austenitic grades, duplex grades exhibit a more complex microstructure structure, (approximately 50/50 austenite and ferrite )and have better resistance to Chloride Stress Corrosion Cracking (CSCC), making them preferred for components exposed to sea water, fertilizers, petrochemicals, and chemicals used for pulp and paper manufacture.
Welding PH Stainless Steels
Arc welding techniques are commonly used for martensitic and semi-austenitic precipitation-hardenable stainless steels grades, with filler metal used to provide control of the fusion zone.
Preheating is not needed in order to avoid cracking. However, stress concentrations such as notches or partial penetration welds should be avoided.
PH grades are usually welded in the solution-treated or annealed condition. Welding in an overaged condition can help mitigate cracking if components are highly restrained (usually in thick sections) and have risk of cracking. The higher Al content found in 15-7 grade can degrade penetration and enhance weld slag formation during arc welding, when compared to more common PH grade 17-4.
When welding with a focused energy beam (EB or laser), heat input and weld cooling rate will be somewhat lower than associated with arc welding. The low heat input results in a much narrower heat affected zone, and different distribution of precipitates in the weld fusion zone microstructure. Mechanical strength of welds that are not subjected to post-weld heat treating (aging) may be lower than adjacent base metal if the parts being welded are already in the aged condition. Joints welded when material is in solution condition can be direct aged to develop higher strength.
When applying laser beam welds, inert gas shielding practice is critical to avoid porosity, usually a result of the presence of minor level of Nitrogen or moisture. Highest purity shield gas is needed. Full penetration joints must be provided with appropriate root side backing gas to prevent contamination.
Electron Beam and Laser welding of PH stainless steels requires a precise joint in order to maintain permissible gap and avoid mismatch. Good weld fixturing is necessary so that welds can be accurately placed, minimizing HAZ areas.
- Butt Weld:
- A fit-up tolerance of 15% of the material thickness is desirable.
- Sheared edges are acceptable provided they are straight and square.
- Misalignment and out-of-flatness of parts should be less than 25% of the material thickness.
- Lap Weld (burn-through or seam weld):
- Air gaps between pieces to be Lap Joint welded severely limit weld penetration and/or feed speed.
- For round welds in PH stainless, no gap can be tolerated unless inert gas coverage can be maintained over the entire weld area.
- Fillet Weld:
- Square edges and good fit-up are also necessary.