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SOLUTIONS: Stainless Steel

Introduction

Stainless steel belongs to the high alloy steel group.

Chromium (Cr) is the element that makes stainless steel stainless or corrosion resistant. The chromium combines with oxygen and creates an invisible though impregnable chromium oxide on the surface. It must contain at least 11,5% chromium, that is the content at which an effective layer is formed that seals the surface against any further atmospheric corrosion. Many stainless alloys contain larger amounts of chromium for further improving corrosion resistance and to increase resistance to oxidation at high temperatures.

Nickel (Ni) in excess of about 6% increases corrosion resistance slightly and greatly improves mechanical properties. Small amounts of Molybdenum (Mo) increase resistance to pitting type corrosion and general resistance to certain corrosive media. Stainless steel alloy with molybdenum is therefore referred to as acid resistant. Molybdenum also improves high temperature strength.

Carbon increases strength, but in connection with stainless steel and in amounts above 0,04% it will cause chromium carbide formation (chrome carbide particles) between 430870°C. The chrome carbide cannot combine with oxygen to create chrome oxide. This reduces the corrosion resistance in local areas leading to intergranular corrosion. The carbon content in stainless steel materials and consumables are therefore kept at a very low level.

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By chemical composition or heat treatment, or a combination, the stainless steel receives one of the following micro structures when manufactured: Austenitic, Ferritic, Martensitic or Ferritic/Austenitic. The American Iron and Steel Institute (AISI) established a numbering system to classify the stainless steel alloys. The AISI classifies stainless steel by their metallurgical structures. This system is useful because the structures (Austenitic, ferritic or martensitic) indicates the general range of mechanical and physical properties and weldability.

The following are some characteristic differences between stainless steel and normal steel: 1. Poorer heat conductivity (approx. 40% less). 2. Higher coefficient of expansion (approx. 40% higher). 3. Greater sensitivity to cold working. 4. Occurence of structural changes following welding, (carbides – sigma phase – ferrite). 5. Sensitivity to certain corrosion phenomena, such as strong pitting, or crack formation along or through the crystals (stress corrosion).

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