Tool Steels – Tungsten High-Speed Steels

Topics Covered

Introduction
Overview
Classification
Hardness
Applications

Introduction

High-speed steels are tool steels that find applications in machine tools that have high rates of material removal. Tungsten high-speed steels (group T) and molybdenum high-speed steels (group M) are two types of high-speed steels. These two groups of high-speed steels have similar hardening abilities and other characteristics.

High-speed tool steels are capable of being hardened to 62 - 67 HRC and their hardness can be maintained at service temperatures up to 540°C (1004°F). This makes them suitable for use in high-speed machinery.

The tungsten series include the T1 to T15 class alloys. Tungsten is a good carbide former that prevents grain growth, enhances toughness and increases red hardness and high temperature strength. Tungsten is used in hot forming tool steels and high-speed steels.

Overview

White and Taylor developed the type T1 series of tungsten high-speed steels. In the early 1900s, they discovered that certain steels exhibited red hardness and such steels comprised more than 14% W, about 0.3% V and about 4% Cr. T1 in its earliest form contained about 18% W, 0.68% C, 0.3% V and 4% Cr. An increase in the quantity of vanadium was seen by 1920. The carbon content of most steels also increased to approximately 0.75% over the years.

The most significant alloying elements found in tungsten high-speed steels include carbon, tungsten, cobalt, chromium and vanadium. Tungsten high-speed steels contain 4% chromium. T4 and T15 are the cobalt-base tungsten varieties that contain different amounts of cobalt. The T1 type of tungsten high-speed steels is free of cobalt or molybdenum.

Classification

The American Iron and Steel Institute (AISI) has classified high-speed tool steels into about 40 individual categories. This classification system uses a T for referring to steels in which tungsten the primary alloying element. The letter T is followed by a number which distinguishes each of the tungsten tool steels ranging between T1 and T15.

The composition of the T1 to T15 tungsten high-speed steels are tabulated below:

Designation Compositiona, %
AISI UNS C Mn Si Cr Ni Mo W V Co
T1 T12001 0.65-0.80 0.10-0.40 0.20-0.40 3.75-4.50 0.30 max - 17.25-18.75 0.90-1.30 -
T2 T12002 0.80-0.90 0.20-0.40 0.20-0.40 3.75-4.50 0.30 max 1.0 max 17.50- 19.00 1.80-2.40 -
T4 T12004 0.70-0.80 0.10-0.40 0.20-0.40 3.75-4.50 0.30 max 0.40-1.00 17.50- 19.00 0.80-1.20 4.25-5.75
T5 T12005 0.75-0.85 0.20-0.40 0.20-0.40 3.75-5.00 0.30 max 0.50- 1.25 17.50-19.00 1.80-2.40 7.00-9.50
T6 T12006 0.75-0.85 0.20-0.40 0.20-0.40 4.00-4.75 0.30 max 0.40-1.00 18.50- 21.00 1.50-2.10 11.00-13.00
T8 T12008 0.75-0.85 0.20-0.40 0.20-0.40 3.75-4.50 0.30 max 0.40-1.00 13.25- 14.75 1.80-2.40 4.25-5.75
T15 T12015 1.50-1.60 0.15-0.40 0.15-0.40 3.75-5.00 0.30 max 1.00 max 11.75- 13.00 4.50-5.25 4.75-5.25

aGroup W contains 0.025 max S, 0.20 max Cu and 0.025 max P. Other steels except group W contain 0.03 max S, 0.25 max Cu and 0.03 max P. Where specified, sulfur may be increased to 06 to 0.15% in order to improve machinability of group A, D, H, M, and T steels.

Hardness

Tungsten high-speed steels have good wear resistance and high red hardness. The maximum hardness of group T steels differs according to the carbon content and also the alloy content. A minimum hardness of 64.5 HRC can be imparted to all types of high-speed steels. Types such as T15 can be hardened to 67 HRC as they have high carbide and carbon content (1.55%). Hence T15 is considered as the most wear-resistant steel of the tungsten high-speed steel series. Tungsten high-speed steels comprising more than 1.0% C and 1.5% V produce a high number of wear-resistant hard carbides in the microstructure due to the presence of high carbon and alloy content. Tungsten high-speed steels are deep hardening when they are quenched from their hardening temperature of 1205 to 1300°C (2200 to 2375°F). Solid tools such as cold extrusion punches and broaches with large diameters are made from tungsten high-speed steels. Full hardness is provided for tools with large diameters using an accelerated oil quench.

Applications

The major applications of tungsten high-speed steels are the following:

  • Interrupted-cut applications and delicate tools
  • Cutting tools including hobs, milling cutters, bits, reamers, drills, broaches and taps
  • Production of dies and punches
  • Making high-temperature and high load structural components, e.g., pump parts and aircraft bearings.

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