01. Metastable austenitic stainless steels
When plastically deformed, traditional austenitic stainless steels with 18% chromium and 8% nickel undergo a martensitic phase transformation. This is due to the fact that the austenite phase is not thermodynamically stable at room temperature. As seen in the phase diagram in Fig 1 the phases expected at room temperature (in true equilibrium) are austenite (γ), ferrite (α) and carbides.
However, very few commercial materials are used in their equilibrium state. A very important family of steels are the so called metastable austenitic stainless steels (MASS) that make use of the properties of strain-induced α’-martensite formed during cold work. These belong to the ASTM 301 (EN 1.4310) and ASTM 302 (EN 1.4310NS) types of stainless steels. They resemble ASTM 304 austenitic stainless steels but are richer in carbon (typically in the range 0.08 - 0.10 wt%) and balanced to give a high strength in cold worked condition. The main difference between type 301 and 302 stainless steels is the somewhat higher nickel content (6-8% and 8-10% respectively) and chromium content (16-18% and 17-19% respectively) in the latter. Among applications we find stainless springs and high strength components.
Owing to the formation of α’-martensite a yield strength (Rp0.2) well above 2000 MPa can be attained in severely cold worked strip or wire. This should be compared with the yield strength of annealed type 304 stainless steel that is typically 250 MPa. For instance, the tensile strength (Rm) in ASTM 302mod (1% molybdenum added) cold drawn down to a diameter of 0.15 mm may exceed 2500 MPa (Fig 2).A reduction in diameter from 8 mm to 0.15 mm raises Rm from 1500 to 2500 MPa and Rp0.2 from 1300 to 2200 MPa.
Applications of MASS include many components in cars such as seat belt retractors, windshield wipers, brake springs and valve springs. They are also suitable as contact springs, hinge springs and clamp springs in electric connectors. Compression springs of the type shown in Fig 3 are standard components. A list of applications that is far from complete is shown in Table 1. Fatigue resistance and resistance to stress relaxation are important requirements in many of these applications.
Experience has shown that ASTM 302 mod with an addition of 1% molybdenum to improve pitting corrosion resistance is a suitable choice as dental reamers, surgical needles and guide-wires used to insert stents in the human body. This is due to the unique combination of strength, ductility and corrosion resistance. Furthermore, owing to the resistance to stress relaxation this alloy is used as springs in automatic syringes kept for many years in military stores. A somewhat different application is spacer expanders in car engines where ASTM 201 (EN 1.4372) has demonstrated satisfactory performance. This alloy differs from ASTM 301 and 302 because nickel is partly replaced by manganese.
There are applications where flawless performance requires a high strength spring material that is non-ferromagnetic. EN 1.4369 (18.5% Cr, 7% Ni, 6% Mn) is tailor-made for such applications. This composition guarantees that the magnetic susceptibility is so low (<5×10-3) that it can be regarded as non-ferromagnetic. Manganese plays an important role here because it promotes the formation of ε-martensite, from which EN 1.4369 derives its strength. As opposed to the usual type of α’-martensite this less common type of martensite is non-ferromagnetic.
A rather recent application of MASS is kitchen knives. Traditional martensitic stainless steel knives described in a previous column (June issue, 2017) possess a hardness that may exceed 60 HRC when properly heat treated. Kitchen knives of type 301 (see Fig 4) only reach hardness values just above 40 HRC but are more easily sharpened with simple sharpening tools. Presumably, this has contributed to their popularity.
Table 1. Typical strength values and applications of metastable austenitic stainless steels
|Standards||Tensile strength (MPa)||Applications|
|≤ 2400*||Wire springs|
|≤ 2000*||Strip springs. Formed parts for e. g. diaphragms, electrical connectors and seat belt retractors|
|ASTM 302 mod||1700-2300* (strip)
|Various types of springs, dental reamers, surgical needles, guide-wires|
|1100-1300*||Spacer expanders in car engines|
|EN 1.4369||850-1800*||Springs and other formed parts for non-magnetic applications|
*Tensile strength strongly depends upon process parameters
We have seen that high strength can be achieved by two different types of martensite; Cooling-induced and plastically deformed. Nature offers another means of achieving extremely high strength. This is the topic of my next column on precipitation hardened stainless steels.
This article was first published in Stainless Steel World Magazine in December 2017.