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Mechanical Engineering Design Notes



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Tool Steels

Introduction
Prior to 1870 all lathe tools were forged from plain carbon steel containing about 1% C and 0.2% Mn, remainder iron. These tools rapdidly lost hardness with increasing temperature (above about 200oC) and could only be used to machine steel at slow speeds, less than 0.1 m/s. This steel had to be water quenched to harden which frequently led to cracking.

In 1870 Robert Mushet, who lived at Coleford in the Forest of Dean, England, introduced a steel with 2% C, 1.6% Mn, 5.5% W and 0.4% Cr. This steel would harden on air cooling and retained its hardness to higher cutting temperatures. This could be used at higher cutting speeds, up to about 0.13 m/s. Developments that followed involved replacing Mn with Cr.

In 1901, Taylor and White introduced tools with greater resistance to softening which could be used at cutting speeds up to about 0.32 m/s. This material (1.9% C, 0.3%Mn, 8% W and 3.8% Cr. was referred to as high speed steel (HSS). This was not a new material, but a new heat treatment for the existing material. Taylor and White found that if the steel was heated quickly through the brittle temperature range of 845 to 930oC to a temperature just below the melting point of the steel before quenching, a better hot hardness was developed.

There were rapid developments during the next few years, Taylor found that better tools were produced when using less C and more W (forgeability was improved). By 1903 the C content of HSS had fallen to 0.7% and the W content risen to 14%.
In 1904 J A Metthews found that the abrasive resistance of HSS could be improved by additions of V and by 1906 Taylor was using 0.3% V in his tools.
By 1910 the W content had risen to 18%, the Cr content to 4% and the V content to 1%. This became known as 18-4-1 HSS which was the standard specification for 40 years (designated: AISI T-1). By 1910 the cutting speed has reached 0.52 m/s.

Electric furnaces for melting were introduced in 1907, facilitating the production of better quality steels, however these furnaces did not come into wide use for 10 years.
In 1912 it was found that the addition of 3-5% Co would improve the red hardness of HSS, but these materials were not widely used as machine tools at that time were still not able to make full use of 18-4-1.

Other formulations with larger quantities of W and Co were investigated, but were not developed commercially till later owing to processing difficulties.
It was known that Mo could replace W (half the weight of Mo was needed as the atomic weights are approximately in the ratio 1:2) but these steels have a narrower heat treating temperature range and a greater tendency to decarburise, so it was not until shortages of W in World War 2 occurred that Mo steels came into wide use. In the USA, the M-2 specification:

0.8% C, 4% Cr, 2% V, 6% W, 5% Mo

replaced T-1 as the standard HSS around 1950.

The hardness and wear resistance of HSS depends upon the composition, size and distribution of the carbides in the steel and on the stability of the matrix at high temperatures. Harder mixed carbides are provided by increased C and V content. Stability of the matrix at high temperatures is increased by the addition of Co.

In the 1950s it was discovered that HSS with a hardness of 70Rc (as opposed to the previously obtained 65Rc) and reasonable toughness could be obtained by the use compositions such as:

1.4% C, 4% Cr, 4% V, 9% W, 4% Mo, 12% Co.

These steels have a large concentration of fine and uniformly divided carbides in a somewhat refractory matrix. The substitution of Mo for W aids forgeability but they are difficult to grind.

An important development in the 1970s was the use of pre mixed alloy powders and hot isostatic compaction to produce alloy compositions that could not be produced by melting and casting.

Classes of HSS and Die Steels.
Type Spec. US
UK
Comp. (%) Heat treatment, oC and hardness Rc Uses
Tungsten high-speed steel T1,
BS4659: BT1
0.75 C
4.25 Cr
18 W
1.2 V
Double pre-heat: 600 and 850
Austenitise: 1290 - 1320
Quench into molten salt, oil, or air blast
Double or triple secondary harden / temper at about 565 for about 1 hour
Rc: 60 - 65
Cutting tools, particularly those likely to face shock load, ie: interrupted cutting
Molybdenum high-speed steel M2,
BS4659: BM2
0.83 C
4.25 Cr
6.5 W
1.9 V
5 Mo
Double pre-heat: 600 and 850
Austenitise: 1250
Quench into molten salt, oil, or air blast
Double or triple secondary harden / temper at about 565 for about 1 hour
Rc: 60 - 65
Tougher than equivalent 18-4-1 tungsten HSS steel. Cutting tools, drills, punches, cold forging dies.
Cobalt high-speed steel T6,
BS4659: BT6
0.8 C
4.75 Cr
20 W
1.5 V
0.5 Mo
12 Co
Double pre-heat: 600 and 850
Austenitise: 1300 - 1320
Quench into molten salt, oil, or air blast
Double or triple secondary harden / temper at about 565 for about 1 hour
Rc: 60 - 65
Has good red hardness and toughness. Tools for severe machining duties, eg on high tensile steels and cast irons.
Chromium hot-work steel H12,
BS4659: BH12
0.35 C
5 Cr
1.35 W
0.4 V
1.5 Mo
1 Si
Preheat: 800
Austenitise: 1300 - 1320
Quench into molten salt, oil, or air blast
Double secondary harden / temper at about 500 to 600 for about 1 hour
Rc: 38 - 55
Suitable for hot die work. Low carbon content means tools can be water cooled without cracking. Extrusion dies, mandrels, hot forming and hot pressing tools
High-carbon, high-chromium, cold-work steel D3,
BS4659: BD3
2.1 C
12.5 Cr
0.3 Mn
1 max W
1 max V
Preheat: 815
Austenitise: 925 - 980
Quench into molten salt, oil, or air blast (thin sections only)
Double secondary harden / temper at about 150 to 400 for about 1 hour
Rc: 54 - 61
Dies for blanking, forming, thread rolling. Rolls, slitter knives. Dies for moulding abrasive powders, eg ceramics.

David J Grieve, 29th September 2004.

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Please contact me for comments and / or corrections or to purchase the book, at: davejgrieve@aol.com