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Most of the commercial standardized austenitic CrNi-stainless steel grades are fully austenitic in annealed condition
especially in the thinner gauges of the cold-rolled sheets and strips.
However, the austenite (γ) is meta-stable and it will transform more or less to
α´ and ε martensite through cold-deformation,
e.g cold-rolling, strech-forming and in tension test.
The fraction of γ transformed into martensitic phases increases at lower temperatures.
The temperature for a given transformed fraction will decrease with increasing alloying content respectively.
γ will decompose into α´ even without any coldworking below the
Ms. All the standard austenitic grades have Ms below roomtemperature.
Because cold-working promotes decomposition of
γ, the martensite starts to form above Ms. All the alloying elements stabilize γ phase. There is the widely used formula by Angel to calculate the temperature Md30 at which an amount of 50 % austenite will be transformed to martensite through cold-deformation of 0.30 true strain (corresponding ca. 35% engineering strain). The newer formula was presented by Nohara 1). I prefere the use of the later one. It takes also the grainsize in account. |
ε martensite is formed first by a cold-forming process and it has a hexagonal crystal-form.
α´ martensite is formed through higher deformation by cold-forming.
It is also a ferro-magnetic phase and it has a cubic body-centered crystal-form.
The martensite phases have a strong influence on the strength properties and on the formability of the austenitic CrNi-stainless steels. α´ and ε increase the coldwork-hardening of γ. It will also increase the uniform elongation through preventing an early necking and this is just a beneficial feature in stretch-forming. One may alternatively get the optimized formability through temperature control during forming operation of a given steel composition while martensite transformation depends highly on the forming temperature. |
| 1) Nohara formula Md30 = 551 - 462*%C - 9.2*%Si - 8.1*%Mn - 13.7*%Cr - 29*%Ni - 18.5*%Mo - 29*%Cu - 68*%Nb - 462*%N - 1.42*(GS[ASTM] -8.0) Ref. Nohara, K., Ono, Y. Ohashi, N. Composition and grain size dependencies of strain-induced martensitic transformation in metastable austenitic stainless steels. J. of ISIJ 63(1977)5, pp 212 ... 222 | |