Crankshaft Material Arma Steel

[beerman]

There is a V6 crankshaft listed on Ebay with a pic of "Arma Steel" cast into the crank (see pic). Wasn't sure what it was so I searched the web and found the article below figured I'd share it before it gets lost to time.


Many years ago, the General Motors (GM) metalcasting division developed an alloy known as Armasteel for use in differential parts, engine crankshafts, automatic transmission stator shafts, connecting rods and other components.

Armasteel served the automaker well in many critical areas. In the GM Corvair, for example, it was used for differential ring gear carrier/cover assemblies. The material also was used for the crankshaft in the high performance 421-cu.-in. Pontiac engine and the connecting rods in the lightweight Buick/ Oldsmobile V-8 engines of the 60s.

However, confusion about the nature of this material has recently emerged in the casting community. Much of this results from GM's marketing efforts to apply a certain mystique to standard engineering materials. First, Armasteel is not steel but a GM trade name for a grade of pearlitic malleable cast iron. As a 1982 GM brochure explains, it is a ferrous alloy with temper carbon in a matrix of tempered pearlite or tempered martensite. It was produced at the GM Central Foundry Saginaw Malleable Iron plant (SMI), which was closed in mid-2007.

A brief history of the plant was given in the article "A History of Innovation" in the Jan. 2001 issue of MODERN CASTING. According to the article, "SMI was one of the first to develop a melting technique in which cupola iron was transported to electric arc furnaces and 'duplexed,' or treated for metallurgical control and held until it was needed at the molding lines. SMI also developed and introduced pearlitic malleable iron (known as Armasteel) in 1936, which was used extensively for military applications during WWII and the Korean War and also for the conversion of crankshafts and connecting rods from steel forgings to cast components in the 1950s."
As heat treatment technology was refined and moved from coal fired batch ovens to continuous gas or electric atmosphere controlled ovens, so did the manufacturing approach and available grades of Armasteel. The resulting four Armasteel grades listed in the GM brochure were referenced by hardness and strength ranges (Table 1).

Beyond the heat treatment to convert the white iron cementite to a temper carbon form, these specific grades were made by either a controlled air quench and temper from the malleablizing temperature (resulting in a tempered pearlite) or a second heat treat operation consisting of reheating followed by an oil quench and temper (tempered martensite).

The confusion over Armasteel also arises from malleable iron becoming less popular over the years. As ductile (nodular or spheroidal graphite) cast iron started to gain acceptance as an engineering material, much of the former malleable iron work (malleable dates back to about 1900, while ductile was invented in 1948) was converted to ductile cast iron. Malleable iron is more difficult to cast and requires the lengthy heat treatment operation to create the temper carbon. Ductile iron can be made directly from magnesium as an alloy treatment addition to the base iron. In addition, most of these Armasteel grades now can be made as-cast with alloying additions of copper or manganese to the base metal.

If you are looking for a replacement alloy for Armasteel, look no further than standard ferrous materials. Any malleable cast iron produced with proper controls will be similar, if not identical, to Armasteel. Ductile iron equivalents also are available for all four grades of the alloy.

Portions of this article were taken from a letter by Bob Anderson, car enthusiast.

Recommendations are the opinion of the AFS Technical Dept. based on referenced literature and experience. If you need assistance with a technical issue, fax or email your question to: Casting Answers & Advice, c/o MODERN CASTING, at 847/824-7848 or moderncasting@afsinc.org.

Table 1. The Four Grades of Armasteel

Tensile Yield
Strength Strength Elong. Hardness
Material (psi) (psi) % (Bhn)

88M 105,000 85,000 2% 269-302

85M modified 90,000 60,000 3% 217-269

85M 80,000 60,000 3% 197-241

86M 70,000 48,000 4% 163-207

Typical
Material Application

88M Gears (high strength &wear resistance)

85M modified Transmission gears

85M Planet carriers

86M Compressor crankshafts

[jbgroby]

Interesting history - Thanks. My flywheel has the name on it as well. I did hear that GM used the metal during World War II, it has heavy specific gravity per pound, tough stuff.

[Jim A]

I hope there won't be a test on that.