Steels_ Metallurgy and Applications, Third Edition

302 Steels: Metallurgy and Applications

Table 4.2

grades

Nominal composition of stainless steel

AISI Type Nominal composition

410

420

430

434

302

304

316

321

347

310

12.5% Cr, 0.15% C max.

13% Cr, 0.15% C min.

17% Cr, 0.12% C max.

17% Cr, 1.0% Mo

18% Cr, 9% Ni

19% Cr, 10% Ni

17% Cr, 12% Ni, 2.5% Mo

18% Cr, 10.5% Ni, Ti min. 5 x C%

18% Cr, 11% Ni, Nb rain. = 10 x C%

25% Cr, 20% Ni

for stainless fiat products was BS 1449: Part 2 Stainless and heat resisting grades,

but this has now been superseded by the following European specifications:

BS EN 10088-1:1995

BS EN 10088-2:1995

BS EN 10088-3:1995

List of stainless steels

Technical delivery conditions for

sheet/plate and strip for general purposes

Technical delivery conditions for

semi-finished products, bars, rods and

sections for general purposes

The last specification for stainless long products also supersedes those covered

earlier in BS 970: Part 1: 1991. As indicated in Table 4.3 BS EN 10088-1

includes 20 ferritic grades, 20 martensitic grades (including precipitation-

hardened steels), 37 austenitic grades and 6 austenitic-ferritic (duplex) grades.

It also contains useful data on the physical properties of these materials. BS EN

10088-2 and 10088-3 present details of the mechanical properties.

Martensitic stainless steels

A fully martensitic structure can be developed in a stainless steel provided:

1. The balance of alloying elements produces a fully austenitic structure at the
   solution treatment temperature, e.g. 1050"C.


2. The Ms-Mr temperature range is above room temperature.

As indicated earlier, these conditions are met in the case of a 12% Cr 0.1% C

steel and such a grade (Type 410) defines the lower bound of composition in

the commercial range of martensitic stainless steels. However, given that the Ms

temperature of a 12% Cr 0.1% C steel is substantially above room temperature,

there is the facility to make further alloy additions and still maintain the marten-

sitic transformation range above room temperature. Therefore the alloy content

can be increased in order to obtain the following improvements in properties:

1. A higher martensitic hardness by means of an increased carbon content.