Wednesday, January 20, 2010
Stainless steels, Cast Iron- useful information for candidates attending NDT Training
Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:
Martensitic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high hardenability, and require both pre- and postheating when welding to prevent cracking in the heat-affected zone (HAZ).
Ferritic stainless steels have 12 to 27 percent chromium with small amounts of austenite-forming alloys.
Austenitic stainless steels offer excellent weldability, but austenite isn’t stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen.
Special properties, including corrosion resistance, oxidation resistance, and strength at high temperatures, can be incorporated into austenitic stainless steels by adding certain alloys like chromium, nickel, molybdenum, nitrogen, titanium, and columbium. And while carbon can add strength at high temperatures, it can also reduce corrosion resistance by forming a compound with chromium. It’s important to note that austenitic alloys can’t be hardened by heat treatment. That means they don’t harden in the welding HAZ.
The carbon content of cast iron is 2.1 percent or more. There are four basic types of cast iron:
1. Gray cast iron, which is relatively soft. It’s easily machined and welded, and you’ll find it used for engine cylinder blocks, pipe, and machine tool structures.
2. White cast iron, which is hard, brittle, and not weldable. It has a compressive strength of more than 200,000 pounds per square inch (PSI), and when it’s annealed, it becomes malleable cast iron.
3. Malleable cast iron, which is annealed white cast iron. It can be welded, machined, is ductile, and offers good strength and shock resistance.
4. Ductile cast iron, which is sometimes called nodular or spheroidal graphite cast iron. It gets this name because its carbon is in the shape of small spheres, not flakes. This makes it both ductile and malleable. It’s also weldable.