The most common grades of stainless steel are 304 and 316, which are particularly popular because their austenitic microstructure results in an excellent combination of corrosion resistance, mechanical and physical properties and ease of fabrication.
The austenitic structure is the result of the addition of approximately 8–10% nickel. Other elements used as an austenite former are manganese, nitrogen, carbon and copper.
The cost of additions
The cost of common stainless steel is substantially determined by the cost of ingredients.
While the cost of the essential ingredient chromium is not high, additions of elements to improve corrosion resistance (especially molybdenum) or to modify fabrication properties (especially nickel) add much to the cost.
Costs for nickel have fluctuated from US$5,000 or US$6,000 per tonne in 2001 to US$15,000 per tonne in 2004.
Similarly, molybdenum has dramatically increased from approximately US$8,000 per tonne in 2001 to around US$50,000 per tonne in 2004.
These costs impact directly on the two most common grades: 304 (18%Cr, 8%Ni) and 316 (17%Cr, 10%Ni, 2%Mo). Most affected is Grade 316, suffering an increase to its cost premium above 304.
Other grades such as the duplex 2205 (22%Cr, 5%Ni, 3%Mo) and more highly alloyed stainless steels are also affected.
Alloying elements achieve changes to the corrosion resistance or to the microstructure, which in turn influences mechanical and fabrication properties.
A common evaluation of corrosion resistance of stainless steel grades is the Pitting Resistance Equivalent (PRE), usually evaluated as PRE = %Cr + 3.3 x %Mo + 16 x %N.
The PRE gives a guide to ranking of grades, but is not a predictor of resistance to any particular corrosive environment. It is apparent that molybdenum can increase pitting corrosion resistance, but so can chromium or nitrogen additions, which are much cheaper than molybdenum.
Despite its high PRE factor, nitrogen has limited effect on corrosion resistance because of low solubility of less than 0.2%. The balance between austenite former elements and ferrite former elements largely determines the microstructure of the steel.
For austenite formers, carbon, manganese, nitrogen and copper are all possible alternatives to nickel. All these elements are lower cost than nickel.
As is the case for the PRE, Ni-equivalence formulas are a guide but do not tell the full story. Each element acts in slightly different ways, and it is not possible to fully remove nickel and replace it with, for example, copper or nitrogen.
Manganese acts as an austenite former but is not as effective as nickel, and Cr-Mn steels have higher work-hardening rates than do apparently equivalent Cr-Ni steels. Carbon is a very powerful austenite former but has only limited solubility in austenite, so is of limited value in a steel intended to be fully austenitic.
Although not recognised by the PRE formula, nickel has positive effects on resistance to some corrosive environments that manganese does not duplicate.
The last decade has seen the rise of new contenders in the Cr-Mn-Ni austenitic group.
The main development work has been in India and the principal application in kitchenware, such as cooking utensils.
The high work-hardening rate of low nickel/high manganese grades has been acceptable to some extent in this application, but the addition of copper can also reduce this problem.
Other Asian countries have also become strong markets and, more recently, producers. The Chinese market is particularly strong, with substantial demand for the Cr-Mn-Ni grades.
*Peter Moore is Technical Services Manager of Atlas Specialty Metals. This edited article courtesy of Australian Stainless Steel Development Association (ASSDA) 07 3220 0722.