Stainless Steel - Mechanical Properties

Topics Covered

Introduction
Typical Properties
Yield Strength
Ductility
Hardness
Mechanical Properties of Wire and Bar

Introduction

The mechanical properties of stainless steels are always the major product specifications when purchasing the product. The properties usually specified for flat rolled products are Brinell or Rockwell hardness, elongation, yield stress and tensile strength. The requirements for Izod or Charpy impact resistance are much less frequent. Yield stress and tensile strength are required for pipe, tube, bar products and fittings. These properties enable manufacturers to calculate the working pressures or loads of the products.

Typical Properties

The mechanical properties of annealed stainless steels are shown in the graph of Figure 1. It should be noted that actual properties of some commercial austenitic steel grades could be enhanced with a high cold-work hardening rate. Even small amounts of cold-work can increase the yield stress.

Figure 1. llustrates the typical mechanical properties of annealed stainless steels.

Yield Strength

A rare property of annealed austenitic stainless steels is that the yield strength is only 40-45% of the tensile strength. However, yield strength can be increased through a small amount of cold-work, thereby simultaneously increasing tensile strength. In severely cold-worked materials, such as strips or spring temper wires, yield strength is usually 80-95% of the tensile strength.

Low yield strength of austenitic stainless steels indicates that the design load of stainless steels cannot be higher than that of mild steel. Design stresses for different temperatures and grades can be referred to from Australian Standard AS1210, "Unfired Pressure Vessels".

Ductility

The other mechanical property of note is the ductility, usually measured by % elongation during a tensile test. This shows the amount of deformation a piece of metal will withstand before it fractures. Austenitic stainless steels have exceptionally high elongations, usually about 60-70% for annealed products, as shown in Figure 2. It is the combination of high work hardening rate and high elongation that permits the severe fabrication operations, which are routinely carried out, such as deep drawing of kitchen sinks and laundry troughs.

Ductility is another important property of the stainless steel that is usually measured in terms of elongation during tensile test. This property is a measure of the ability of a metal to withstand deformation before it fractures. Austenitic stainless steels exhibit high elongations of about 60-70% for annealed products. Fabrication operations, such as deep drawing of laundry troughs and kitchen sinks, can be carried out at extreme conditions with respect to high elongation and work hardening rates.

Figure 2. illustrates the typical elongations of annealed stainless steel materials.

Hardness

Hardness, generally measured using Vickers, Rockwell or Brinell machines, is another property that represents the material’s strength. It is defined as a penetration resistance, and the test machines measure the depth of a material into which a very hard indenter is pushed by means of a known force. Each machine consists of a different force application system, as well as a different shaped indenter and, hence, conversion between hardness scales is not very accurate. Conversion tables for hardness have been created. These conversions are approximate and should not be used for determining conformance to standards.

Sometimes, performing a hardness test is very convenient, such that the result can be easily converted into tensile strength.

Mechanical Properties of Wire and Bar

Tensile strength precisely describes the mechanical properties of a number of stainless steel bar and wire products. These products require carefully chosen mechanical properties for efficient fabrication into the finished component, and also to tolerate the loads applied during operations. Spring wire has the highest tensile strength of the wire, which is ideal for coiling it into compression or tension springs without damaging during forming. However, such high tensile strengths would not be suitable for weaving and forming applications, as the wire tends to break on forming.

Weaving wires are provided in a number of tensile strengths carefully chosen, such that the finished woven screen will have sufficient strength to tolerate service loads. Wires used in the applications of fasteners also require a proper balance of mechanical properties. The wire used in fasteners must be ductile enough to develop a complex head, and at the same time it needs to be hard enough to avoid deformation threads while assembling screws or bolts into the component. Composition of the steel needs to be considered to achieve mechanical properties of components, such as self-tapping screws, wood screws and roofing bolts.

For bar products, a large proportion is usually machined so as to achieve better load carrying capacity. A good, bright finish for a drawn bar can be achieved by increasing the strength of the products.