Ni-Cr-Mo Alloys

A: This is not an easy question to answer since each alloy has its own strengths and weaknesses. Ni-Cr-Mo alloys are best known as the Hastelloy C type alloys in the wrought form, but they are less recognizable when their ASTM cast designations are used. These C type alloys are nickel-based and contain 15% to 24% chromium and 12.5% to 20% molybdenum with the percentages of chromium and molybdenum a little different for each alloy. Some of the C type alloys also contain small percentages of tungsten, vanadium or copper.

Confusion over which grade is best stems from the fact there are so many grades of the C type alloys. In the wrought form, there is UNS N10276 (Hastelloy C-276), UNS N06455 (Hastelloy C-4), UNS N06022 (Hastelloy C-22), UNS N06200 (Hastelloy C-2000), and then there is UNS N06059 (Alloy 59) from ThyssenKrupp VDM. For castings, ASTM specification A494 for nickel-based castings gives us CW12MW, CW6M, CW2M, CW2MW and CX2M. This expanse of choices makes it difficult for a user to know what grade they really need. Likewise, for suppliers, it is not good business practice to stock or to offer all these grades. Let’s look at each alloy individually.

CW12MW is the old original cast Hastelloy C alloy. Its higher carbon limit of 0.12% maximum along with its higher iron and tungsten levels makes this alloy inferior to any of the other grades in corrosion resistance and ductility. CW12MW is not the cast equivalent for UNS N10276.

CW6M is the next cast Ni-Cr-Mo alloy developed. It was introduced in 1949 so there is a long, successful history for this alloy. While it does have a higher allowable carbon and silicon limit than the newer C types listed below, it is typically made with much lower carbon and silicon levels, thus giving it good corrosion resistance to both oxidizing and reducing acids.

CW2M is the cast equivalent of UNS N06455 developed in 1976 to provide better metallurgical stability than UNS N10276 at high temperatures. An added benefit was that it is a better cast grade than CW12MW. Its improved performance over CW12MW was a result of reduced amounts of intermetallic phases in its microstructure. This was accomplished by lowering its carbon, silicon, iron, chromium and molybdenum levels and eliminating the tungsten addition.

CX2MW is the cast equivalent of UNS N06022. This alloy has a higher chromium content, but a lower molybdenum content than the other Hastelloy C type alloys. The higher chromium content does give this alloy better corrosion resistance to hot oxidizing environments such as bleach and chlorinated brines. However, the lower molybdenum content sacrifices some resistance to reducing services where these alloys are also commonly used, e.g., sulfuric and hydrochloric acids.

CX2M is the cast equivalent to UNS N06059. It has slightly higher chromium and molybdenum than CX2MW. The higher chromium can provide good resistance to strong oxidizing services and its higher molybdenum content can provide good resistance to sulfuric and hydrochloric acids.

UNS N06200 (Hastelloy C-2000) is the newest C type alloy, but it is only available as a wrought component because its extremely low carbon and silicon content mean it’s not beneficial for making a cast alloy. UNS N06200 contains a chromium content similar to UNS N06022, but it has more molybdenum along with a small amount of copper. The higher molybdenum content along with the copper gives the alloy better resistance to reducing acids like sulfuric and hydrochloric.

CONSIDER CASTING ­QUALITY

So which grade is better overall? The answer cannot be found without first considering casting quality. These are fairly complex alloys, and if their chemistry and heat treatment are not optimized, their corrosion resistance will not be sufficient. A widely held—but mistaken—belief is that if an alloy meets the ASTM requirements, it must be a good alloy. For most alloys and in particular these higher, more complex alloys, an optimum chemistry and heat treatment will give the alloy its best corrosion resistance. Therefore, it takes a competent foundry to produce the best quality C type alloy.

In summary, no one alloy will perform the best in both reducing and oxidizing environments. Generally, the higher chromium grades will do better in oxidizing conditions and the higher molybdenum alloys will do better in reducing environments. Also, keep in mind that the performance of any alloy is dependent upon the competency of the foundry producing it, and that is particularly true for these high nickel base alloys.

THOMAS SPENCE is director of materials engineering for Flowserve Corp. (www.flowserve.com), Dayton, OH. Reach him at tspence@flowserve.com.