Tool Steels – Air-Hardening, Medium Alloy, Cold Work Steels

Topics Covered

Introduction
Overview
Composition
Properties
Applications

Introduction

Cold-work steels are high carbon steels containing low alloy additions of molybdenum, tungsten, chromium and manganese. The additions of these alloys allow oil quenching with less distortion and they also increase hardenability.

Cold-work steels have high wear resistance and are used at temperatures of up to 200°C. Excellent toughness characteristics are exhibited by cold-work steels based on their specific usage areas. Their alloy compositions are designed in a manner to provide excellent dimensional stability and moderate-to-high hardenability. At temperatures above 205 to 260°C (400 to 500°F), these steels may undergo softening as they have low alloy contents.

The applications of cold-work steels include plastic molds, threading dies, knurling tools, gages, drawing and piercing dies, blanking, forming and banding rolls, broaches, mandrels, taps, reamers, shears, centers and lathe.

Overview

Air-hardening, medium alloy, cold work steels or Group A steels include types A2, A3, A4, A6, A7, A8, A9, and A10. Sufficient amounts of molybdenum, chromium and manganese are present in group A steels in order to obtain full hardness in sections having a thickness of at least 100 mm (4 in.) in diameter when air cooled. Group A steels have a carbon content ranging from 0.5% to 2%. Group A tool steels possess air hardening characteristics that allow minimum cracking and distortion on quenching.

Air-hardening, medium-alloy cold work tool steels are further divided into two groups as follows:

  1. Group 1: A2, A3, A8, and A9.
  2. Group 2: A4, A6, A7and A10.

Group 1 air-hardening steels have high chromium content of 5%. These steels develop moderate softening resistance at high temperatures.

Group 2 air-hardening cold work tool steels comprise high levels of manganese and exhibit enhanced hardenability. The carbon content of steels on this group is lower in comparison with that of group 1. These tool steels can be hardened from about 110°C (200°F), thus reducing unwanted surface reactions and distortion during heat treatment.

Composition

The designation and composition of group A tool steels are tabulated below.

Designation Composition, %
AISI UNS C Mn Si Cr Ni Mo W V Co
A2 T30102 0.95-1.05 1.00 max 0.50 max 4.75-5.50 0.30 max 0.90-1.40 - 0.15-0.50 -
A3 T30103 1.20-1.30 0.40-0.60 0.50 max 4.75-5.50 0.30 max 0.90-1.40 - 0.80-1.40 -
A4 T30104 0.95-1.05 1.80-2.20 0.50 max 0.90-2.20 0.30 max 0.90-1.40 - - -
A6 T30106 0.65-0.75 1.80-2.50 0.50 max 0.90-1.20 0.30 max 0.90-1.40 - - -
A7 T30107 2.00-2.85 0.80 max 0.50 max 5.00-5.75 0.30 max 0.90-1.40 0.50-1.50 3.90-5.15 -
A8 T30108 0.50-0.60 0.50 max 0.75-1.10 4.75-5.50 0.30 max 1.15-1.65 1.00-1.50 - -
A9 T30109 0.45-0.55 0.50 max 0.95-1.15 4.75-5.50 1.25-1.75 1.30-1.80 - 0.80-1.40 -
A10 T30110 1.25-1.50a 1.60-2.10 1.00-1.50 - 1.55-2.05 1.25-1.75 - - -
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aContains free graphite in the microstructure.

Properties

Silicon is added to types A8, A9 and A10 to enhance toughness. Nickel is also added to types A9 and A10. Graphite is formed in the microstructure due to the high levels of silicon and carbon in type 10, thus enabling A10 to have improved resistance to seizing and galling in a fully hardened state in comparison with other tool steels in group A.

The martensitic matrix provides wear resistance that is enhanced by the complex chromium or chromium-vanadium carbides that are found in air-hardening, medium-alloy cold work tool steels. This in turn enhances the performance of the steels in harsh conditions without achieving complete hardness. Full hardness can be achieved by interrupted quenching in hot oil or by cooling larger sections of the group A steels in an air blast.

Applications

The applications of air-hardening, medium alloy, cold work steels are the following:

  • Coining dies, forming dies, shear knives, blanking and trimming dies and punches
  • Precision measuring tools and gages
  • Highly abrasive applications, ceramic and brick molds.

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