VOLUME 22

ISSUE 2
of Achievements in Materials June
and Manufacturing Engineering 2007

Austenite stability in the high strength

metastable stainless steels
S.J. Pawlak*
Institute for Ferrous Metallurgy, ul. K. Miarki 12, 44-100 Gliwice, Poland
* Corresponding author: E-mail address: spawlak@imz.gliwice.pl
Received 27.03.2007; published in revised form 01.06.2007

Manufacturing and processing

Abstract
Purpose: The aim of the present paper was to study the peculiarities of the austenite to martensite phase
transformation (A-M), which is an essential step in the production technology of the high strength metastable
stainless steels.
Design/methodology/approach: The desired control over A-M transformation have been achieved by proper
design of the steel chemistry, cold working and heat treatment.
Findings: For a range of steel compositions, it was shown that severe cold working leads to fully martensitic
structures. Alternatively, the deep refrigeration treatment or heating at about 750°C, are also effective in that
respect. Subsequent ageing treatment of the deformed martensite, or martensite obtained by subzero treatment,
develops interesting set of the mechanical properties.
Research limitations/implications: To fully evaluate the properties of the steels further research is needed,
particularly involved with the cold drawing and ageing response.
Practical implications: The new metastable stainless steels could achieve properties making them competitive
to some high strength steels and alloys - those with not so good combination of formability, strength and
corrosion resistance
Originality/value: Revealed transformation behavior of the complex alloyed austenites makes an essential step
in development of the high strength metastable stainless steels.
Keywords: Heat treatment; Metastable austenite; Martensitic stainless steels; Cold working

but they may have good combination of the formability, strength,

1. Introduction
1. Introduction ductility and corrosion resistance, which make them competitive
to other high strength steels and alloys. For example, the high
The metastable stainless steels, also known as the controlled- strength quenched and tempered steels often require surface
transformation stainless steels, try to combine the best properties protection involving polluting agents, and for the environmental
of the austenitic and martensitic steels [1÷5]. They are used reasons the high strength stainless steels are gaining renewed
primarily in the form of sheets, strips and wires. The fabrication interest. When making parts of the high strength stainless steels,
in the sheet form is made in the austenitic state. After annealing stability of the austenite is one of the major concerns. The
the soft austenitic structure is obtained, which transforms to problem of the retained austenite stability has recently been
martensite during subsequent operations such as cold working or studied also in the TRIP-aided low alloy steels [6, 7].
refrigeration. The last operation is ageing, which provides the For several steels of widely varied chemical compositions, the
steel with additional strength. Steel toughness is strongly austenite stability during cold rolling and refrigeration has been
dependent upon particular grade and treatment. These steels are studied, to evaluate the degree of control that could be achieved
not so readily drawn or formed as stable austenitic stainless steels, over microstructure and resultant mechanical properties.

© Copyright by International OCSCO World Press. All rights reserved. 2007 Short paper 91
 Journal of Achievements in Materials and Manufacturing Engineering Volume 22 Issue 2 June 2007

Table 1.
Chemical composition of the stainless steels, 25 kg laboratory melts, [wt.%]
Steel C N Cr Ni Mo Mn Si Al Cu
1H14ANG2SCu(A) 0.10 0.10 14.0 1.5 0.50 2.0 1.5 - 1.5
0H15N6SCuJ(B) 0.07 - 15.0 6.0 - 1.0 1.5 1.20 1.5
1H15N4AM3(N) 0.15 0.10 14.57 5.3 2.32 1.09 0.63 0.12 -
0H15N7M2J(J) 0.05 0.02 15.53 7.03 2.3 0.42 0.28 1.08 -
OH12N8M2J(H) 0.02 0.01 11.67 7.9 2.13 0.20 0.10 0.96 -

2. 
Experimental
2. Experimental x severe cold working,
x heating at about 750qC.
To study the microstructure of the steels, routine
metallographic procedures of optical microscopy and
transmission electron microscopy were used. To measure
austenite content in the cold worked steels, the special X-ray
diffraction procedure was used, in which many peaks intensity
from alpha and gamma phases has been taken into calculation,
to compensate for strong preferred orientation of the grains. The
subzero transformation of the austenite to martensite has been
studied by the dilatation changes using optical dilatometer
equipped with the refrigeration chamber.
The chemical compositions of the 25 kg laboratory melted
steels are given in Table 1. First three steels: 1H14ANG2SCu
(steel A), 0H15N6SCuJ (steel B) and 1H15N4AM3 (steel N),
have high interstitials content and high level of silicon and
manganese. The (A) and (B) steels have particularly lean
specification in terms of expensive elements, such as nickel and
molybdenum. The higher nickel is present in the last two steels,
but silicon, manganese and interstitials levels are much lower, Fig. 1. Location of the metastable stainless steels on the
compared to other alloys. Each steel contains the elements Schaeffler diagram
important for the secondary hardening (Al, Cu, Mo, Ni).

3. Results
3. Results andand discussion
discussion
Depending on a specific composition, Fe-Cr-Ni-X alloys may
have austenitic, martensitic, ferritic or mixed microstructures. The
steels microstructures are conveniently presented with the use of
Schaeffler diagram, Fig. 1. Compositions of the metastable steels
studied in the present work were so balanced that they all fell in
the "M-A" region, between the fully austenitic and martensitic
fields. The diagram also indicates that some small amount of the
delta ferrite should be expected in the steels studied. The
Schaeffler diagram serves essentially to predict weld
microstructure, and may not be necessarily accurate for
microstructures formed by other means.
Fig. 2 shows predominantly austenitic structure with some Fig. 2. Austenitic structure with some amount of martensite in the
amount of martensite in the 1H15N4AM3(N) steel after solution 1H15N4AM3(N) steel after solution annealing at 1050qC (the
annealing at 1050qC. Another example of microstructure in optical micrograph)
annealed state, where the delta ferrite was revealed in the
1H14ANG2SCu(A) steel is shown in Fig. 3(a). The heating at 750qC causes precipitation of the M23C6
For the metastable stainless steels there are three basic ways carbide, producing austenitic matrix depleted of the chromium
to transform the austenite into martensite: and carbon, what effectively rises the Ms temperature.
x refrigeration well below room temperature, Microstructures produced in that way have poor properties, both

92 Short paper S.J. Pawlak

 Manufacturing and processing

in terms of strength and toughnesss [8, 9]. This heat treatment is The isothermal transformation of austenite to martensite at the
of limited value and was not considered any further. subzero temperatures in the 1H16AN5M3 steel is shown in Fig. 4.
The refrigeration heat treatment is often applied to heat The kinetics of transformation has a C-shaped curve, and proceed
treatment of the tool and medium carbon quenched and tempered with greatest rate at the temperatures close to -80qC. The
steels to eliminate the retained austenite [10÷13]. refrigeration treatment should be made within short time after
solution annealing, otherwise the stabilization may occur.
Example of microstructure obtained in the (A) steel after cold
rolling with deformation of 60%, is shown in Fig. 3(b). Apart
from martensite, there are highly deformed ferrite grains but they
were not clearly seen on the transmission electron micrographs.
The effect of cold deformation by rolling on the austenite stability
has been studied for the 1H14ANG2SCu(A), 0H15N6SCuJ(B)
and 0H15N7M2J(J) steels, Fig. 5. The greatest strain hardening
rate is observed in the 1H14ANG2SCu(A) steel which have
highest interstitials content.
This steel developed most of its maximum hardness
(“saturation hardness”) at cold reductions up to 30%. For the (B)
and (J) steels, having lower interstitials content (<0.10%), the
hardness increases more gradually over the range of cold
reductions studied.

Fig. 3. Microstructures of the 1H14ANG2SCu(A) steel showing:

a) į-ferrite (the optical micrograph); b) martensite formed by cold
rolling with deformation of 60% (the transmission electron
micrograph)

Fig. 5. The effect of cold deformation by rolling on the austenite

stability and hardness of the metastable steels (A, B and J)

Table 2 shows results of the tensile test of solution annealed

steels. The 1H15N4AM3(N) and 0H15N7M2J(J) metastable
steels have low yield stress and develop very high strengths after
straining. The observed strength gap between yield stress and
ultimate tensile strength could be as high as 930 MPa. In the
tensile test of the (N) and (J) steels, the high strain hardening rate
is accompanied with large relative elongations - both at fracture,
and as measured at the point of maximum force, Agt (the uniform
elongation). Such large elongations were not observed in the
Fig. 4. Isothermal kinetics of the austenite to martensite 0H12N8M2J(H) martensitic steel of similar resultant strength.
transformation in the 1H16AN4M3 steel at subzero temperatures Apparently at tensile straining of the steels with higher

Austenite stability in the high strength metastable stainless steels 93

 Journal of Achievements in Materials and Manufacturing Engineering Volume 22 Issue 2 June 2007

interstitials content, the austenite transforms to martensite giving References

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