Alloying elements that help stabilize the austenitic phase reduce the tendency of the austenitic stainless steel to work hardening. Nickel additions have been used traditionally to do this, but nitrogen also has a profound affect on stability of the austenitic phase.
Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.
Heat treatment of metals and alloys
Metallic materials consist of a microstructure of small crystals called “grains” or crystallites. The nature of the grains (i.e. grain size and composition) is one of the most effective factors that can determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling rate of diffusion, and the rate of cooling within the microstructure.
Complex heat treating schedules are often devised by metallurgists to optimize an alloy’s mechanical properties. In the aerospace industry, a superalloy may undergo five or more different heat treating operations to develop the desired properties. This can lead to quality problems depending on the accuracy of the furnace’s temperature controls and timer.
The nickel level was increased in the superseded BS1449 grade 304S16 compared to the 304S15 level (around 8.0%). This enabled the 304S16 grade with around 8.5% nickel to be used for deep drawing applications. Both these grades are covered inEN 10088-2 as 1.4301, but the higher nickel variant can be specified as a ‘deep drawing’ grade. In contrast the higher nickel variants of 316 (1.4435) were developed for improved ‘selective’ corrosion resistance, originally in pharmaceutical applications, from lower ferrite levels. The ‘standard’ 316 type (1.4401) with around 11% nickel should be suitable for deep drawing.
The BS1449 grade 305S19 with its 11.0 – 13.0 % nickel range is even more stable when cold worked. One application for this grade is for temper rolled strip for springs where low magnetic permeability is required. EN 10088-2 covers this grade as 1.4303. An alternative is the Sandvik strip grade ’13RM19′ with 6% manganese and 0.25% nitrogen in addition to 7% nickel.
Stretch forming application grades would normally have ‘standard’ nickel levels (around 8.0 / 8.2%), but if the stainless steel sheet is intended for stretch forming the manufacturer / supplier should be informed, as the final heat treatment / process line speed conditions are adjusted to optimize the mechanical properties. The grain size of cold rolled stainless steel sheet is usually fine enough (around ASTM 7-8) to avoid ‘orange peel’ surface roughening during pressing.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)