Tool Steels

Plain carbon steels, if used for cutting tools, lack certain characteristics necessary for high-speed production, such as red hardness and hot -strength toughness. The effect of alloying elements in steel is of great advantage and yields tool steels that overcome many of the shortcomings of the plain carbon steels.

Tool steels are defined by U.S. steel producers as "carbon or alloy steels capable of being hardened and tempered". Many alloy steels would fit this loose definition. Tool steels usually contain significantly more alloying elements than alloy steels. However, the real factor that discriminates tool steels from carbon or allot steels is the manufacturing practice.

Approximately 70 types of tool steels are available in the United States. One reason for so many types of tool steels is evolutionary development over a period of 80 years. The second reason is the wide range of needs that they serve. Tool steels have properties that permit their use as tools for cutting and shaping metals and other materials both hot and cold. The U.S. tool steel classifications system is based on use characteristics. There are six major categories one of which contains grades intended for special purposes. A prefix letter is used in the alloy identification system to show use category, and the specific alloy in a particular category is identified by one or two digits. For example:

S1 = Shock resistant tool steel

D2 = Cold-work tool steel

H11 = Hot work tool steel

M42 = High-speed tool steel

 

Tool Steel Type

Prefix

Specific Types

Cold Work

W = Water Hardening

O = Oil Hardening

A = Medium alloy Air Hardening

D = High Carbon, High Chromium

W1, W2, W5

O1, O2, O6, O7

A2, A4, A6, A7, A8, A9, A10, A11

D2, D3, D4, D5, D7

Shock Resisting

S

S1, S2, S4, S5, S6, S7

Hot Work

H

H10-H19 Chromium types

H20-H39 Tungsten types

H40-H59 Molybdenum types

High Speed

M

 

T

Molybdenum types (M1, M2, M3-1, M3-2, M4, M6, M7, M10, M33, M34, M36, M41, M42, M46, M50

Tungsten types (T1, T4, T5, T6, T8, T15)

Mold Steels

P

P6, P20, P21

Special Purpose

L and F series

L2, L6

 

One problem that exists in discussing the metallurgy of tool steels is that, since there are six major categories of tool steels, it is difficult to make statements that apply to all these steels. The following are some of the characteristics of tool steels:

  1. Composition and physical properties vary significantly (some tool steels have compositions that fit into the composition ranges of carbon and alloy steels, but most tool steels have alloy concentrations that are significantly higher than the carbon and alloy steels),
  2. One important factor that should be kept in mind is that the alloy additions do not improve corrosion resistance even though some grades have as much chromium as stainless steels. The reason for this is that alloy elements are usually combined with carbon to form carbides.
  3. The most significant metallurgical difference between tool steels and the other steels is their microstructure. A fully hardened carbon steel or alloy steel would have only martensite as the predominant phase. Most tool steels have a hardened structure of martensite and alloy carbides.
  4. Require special heat treatment processes ,
  5. Higher cost than alloy steels,
  6. Better hardenability than most carbon and alloy steels,
  7. High heat resistance
  8. Easier to heat treat,
  9. More difficult to machine than carbon and alloy steels
  10. Most tool steels are sold as hot-finished shapes such as rounds and bars,
  11. Cold-finished sheets are not available because it is difficult to cold roll or cold finish these materials.

Cold Work Tool Steels (W, O, A, D-types):

Cold work tool steels are used for gages, blanking, drawing and piercing dies, shears, forming and banding rolls, lathe centers, mandrels, broaches, reamers, taps, threading dies, plastic molds, knurling tools.

Water Hardening Tool Steels

(W series)

Oil Hardening Tool Steels

(O-Series)

Medium Alloy Air Hardening Steels

(A-series)

High Carbon High Chromium Steels

(D-series)

Essentially these are carbon steels with 0.60 to 1.10 % carbon.

Lowest cost tool steels.

Soft core(for toughness) with hard shallow layer (for wear resistance).

Use of w-series steels is declining.

0.90 to 1.45 % Carbon with Mn, Si, W, Mo, Cr.

They contain graphite in the hardened structure along with martensite. (Graphite acts as a lubricator and also makes machining easier.

Tungsten forms tungsten carbide which improves the abrasion resistance and edge retention in cutting devices.

5 to 10 % alloying elements (Mn, Si, W, Mo, Cr, V, Ni) to improve the hardenability, wear resistance, toughness.

All D-series contain 12% Cr and over 1.5 % C.

Air or oil quench.

Low distortion, high abrasion resistance.

 

Hot Work Tool Steels (H-series):

There are about 12 hot-worked tool steels. They are categorized by major alloying elements into three subgroups.

Chromium types

Tungsten types

Molybdenum types

These steels are used in extrusion dies, forging dies, die casting, hot shear blades, plastic molds, punches and dies for piercing shells, hot press, etc.

 

Shock Resisting Tool Steels (S-series):

These steels have 0.45 to 0.55 % carbon. The alloys, silicon, and nickel are ferrite strengtheners. Chromium increases wear resistance and hardenability. The S-series of tool steels were originally developed for chisel-type applications, but the number of alloys in this category has evolved to include steels with a broad range of tool applications. This class of steels has a very good shock resistant qualities with excellent toughness.

They are used in form tools, chisels, punches, cutting blades, springs, trimming, and swaging dies, concrete and rock drills, bolt cutters.

 

Mold Tool Steels (P-series):

These steels have 0.10 to 0.35 % carbon. They show high toughness. The low carbon mold steels cannot be quench hardened. The carbon and alloy content is low to allow hubbing of mold details. The desired mold shape is pressed into the steel with a hub that is usually made from a high-speed steel. Thus mold cavities can be made without machining. Hubbed cavities are then carburized to make a production injection molding cavity.

High-Speed Tool Steels (M and T-series):

These are the classes of steel that deep harden, retain that hardness at elevated temperatures, and have high resistance to wear and abrasion. The carbon content of these steels vary from 0.85 % to 1.50 %.

M-type:

The M-type tool steels are high in molybdenum content and are used for lathe centers, blanking dies, hot forming dies, lathe cutting tools, drills, taps, etc. They are used in almost all cutting tools.

T-type:

The T-type high speed tool steels with high carbon content have high wear resistance and very high hardness. The ones with lower carbon content are tougher but not as hard as the former group. As the amount of tungsten increases, the toughness decreases. This class of tool material has a substantial amount of wear-resistant carbides in a very high heat resistant matrix. These steels are used in machine cutting tools such as tool bits, milling cutters, taps, reamers, drills, broaches. In some instances it is used where high temperature structural steel is needed.

Special Purpose Tool Steels (L and F series):

The L-type steels are low alloy steels with about 1 % Cr that makes them a good low cost substitute for cold work steels. The F-type steels are high in carbon tungsten. They have high wear resistance, good toughness, and medium hardenability. The L-type steels are used in gages, broaches, drills, taps, threading dies, ball and roller bearings, clutch plates, knurls, files. The F-type steels are used as finish machining tools.They have good wear resistance and will maintain a sharp cutting edge. They may be used in dies, cutting tools, form tools, knives, etc.

 

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Last Update: November 19, 1999

Prepared by: Serdar Z. Elgun