Precipitation hardening stainless steels

These steels do not have ANSI numbers and are referred to by trade name. They can be heat-treated to give the following mechanical properties. 

The corrosion resistance of these alloys is similar to CF8 and 304 and better than the 400-series SSTs. CB7Cu-l and CB7Cu-2 resist atmospheric attack in all but the most severe environments. They are resistant to natural water, except seawater, where pitting can be expected. They are widely used in steam, boiler feed water, condensate, and dry gases. 

When the chemistry of a stainless steel is adjusted properly, both ferrite and austenite will be present at room temperature. SSTs with approximately 50% austenite and 50% ferrite are called duplex SSTs (see Table 14.9). The popularity of these materials has increased rapidly in recent years 

Specification and Grade Wrought Equivalent CMax Cr Ni Other Elements 

Chemical Composition of Cast Duplex Stainless Steel 

Welding of duplex alloys can also be somewhat difficult due to the potential for forming the a phase. Welding filler material containing about 1-2% more nickel than the casting is normally used when the castings will be re-solution heat-treated. Filler material with 3% additional nickel is used when castings are not re-solution heat-treated.  

---

The enhanced strength and corrosion properties of duplex stainless steels depend on maintaining equal amounts of the austenite and ferrite phases. The welding thermal cycle can dismpt this balance therefore, proper weld-parameter and filler metal selection is essential. Precipitation-hardened stainless steels derive their additional strength from alloy precipitates in an austenitic or martensitic stainless steel matrix. To obtain weld properties neat those of the base metal, these steels are heat treated after welding. 

Table 3.12. Examples of Precipitation Hardening Stainless Steels...

The precipitation-hardening stainless steels are proprietary grades hardened by both the martensitic transformation and precipitation hardening. These contain higher amounts of chromium (16-17% ) and nickel (4- 7%) than (he 12% chromium ferritic alloys. These steels are normally used at lower temperatures than the 12% chromium ferritic superalloys. 

In some cases testing may be needed to identify a suitable material. For example, low concentrations of CO2 (250 ppm) in anhydrous hydrazine accelerate the decomposition of hydrazine in stainless steel. Long-term storage tests of hydrazine propellants in 17-7 PH stainless steel and AM350 precipitation-hardened stainless steel at 50°C for about three years showed no pressure rise or hydrazine decomposition253. 

Precipitation hardening stainless steels must never be used in the as-purchased annealed condition. They are not only soft and weak, but they lack toughness and will fail prematurely. 

Figure 4-8. Precipitation hardening stainless steels are the strongest group of stainless steels.

Precipitation hardening stainless steels are those alloys ending with the sufBx PH (i.e., precipitation hardening), for example, 17 PH (17Cr-4Ni-4Cu) (UNS S17400). They are hardenable by heat treatment. They are most often used for springs, valve stems, the internals of rotating equipment. 

The martensitic and precipitation-hardening stainless steels are more susceptible to hydrogen embrittlement than are the austenitic alloys. The susceptibility of these stainless steels is sensitive to microstructure and strength level. Figure 22 shows the resistance to cracking for a martensitic (type 410) stainless steel and some precipitation-hardening stainless steels in an H2S-saturated solution [174]. The low resistance of type 410 stainless steel is typical for most... 

R. R. Gaugh, Sulfide stress cracking of precipitation-hardening stainless steels. 

Ferritic stainless steel, precipitation-hardening stainless steel, chromium, brass, bronze, copper... 

---