Stress Relief Compare to Normalised

Hello FAIZOL82

The two terms are often used incorrectly, and below are listed the two terms from the same Makers, showing the differences between the terms.

Stress relief is achieved by heating the casting to a sufficiently high temperature that its strength is reduced to the extent that the residual stress can be relieved by plastic deformation. The extent to which stresses will be relieved or eliminated is dependent on several factors, including the initial severity of the residual stresses, the stress relieving time and temperature, the heating-cooling cycle, and the composition and microstructure of the casting.

Stress Relieving is a process used for reducing or eliminating stresses in iron castings by reheating and cooling them at a controlled rate. High stress in castings can cause distortion or warping, especially during machining. The most common reasons for high stress in castings are non-uniform section thickness or differences in surface cooling rates and internal cooling rates for the same section.

Uniform or thick section castings normally have low residual stress, while thin or uneven sectioned castings retain higher residual stress. By elevating temperature between 900 and 1200 F and holding for a predetermined time, then allowing the casting to cool at a controlled rate, most if not all stress can be eliminated from the product. Yield strength will not be affected by this process.

Normalizing and Tempering

Castings may be tempered after normalizing to provide an optimum combination of high strength and toughness. This process also provides the additional advantage of improving the control of properties through selection of tempering temperature and time.

Normalizing involves the austenitizing of a Ductile Iron casting, followed by cooling in air through the critical temperature. An as-cast Ductile Iron casting is normalized in order to: break down carbides, increase hardness and strength, and produce more uniform properties (see Figures 3.16 and 3.18). Normalizing should be carried out at an austenitizing temperature approximately 100^oC (212^oF) above the critical temperature range. Typically, austenitizing temperatures in the range 1600-1650^oF (875-900^oC) and holding times of one hour, plus one hour per inch of casting thickness, are adequate to produce a fully austenitic structure in unalloyed castings relatively free of carbide. The cooling rate should be sufficiently rapid to suppress ferrite formation and produce a fully pearlitic structure. Depending on casting section size and alloy content, adequate cooling rates can be achieved in still air, or large fans may be required. If fan cooling cannot produce the desired pearlitic structure, the castings should be alloyed with pearlite stabilizing elements such as copper, tin, nickel or antimony. Figure 7.6 illustrates the effect of alloy content and section size on the hardness of normalized Ductile Iron. Step normalizing, which employs a second, lower temperature stage prior to air cooling, can be used to provide the improved matrix control required for the production the pearlitic/ferritic grades of Ductile Iron.

As you may see from the above, the processes are different, and thus achieve different results.

Trust that assists you.

Kind Regards....

Dear Sir,

Thank for your explaination.