Engineering Materials by Tindell Henry

Author:Tindell, Henry
Language: eng
Format: epub
Publisher: Crowood

Maximum strength (σy ~700–1500 N/mm2, 100–200 ksi)

Apart from a sole Cu-base alloy, precipitation hardened Cu-2 per cent Be, and the α + β titanium alloys, this category is the province of the specialized steels and high-Ni aerospace alloys. The commercial purity titaniums are not capable of joining this category and the other light alloys are left far behind.

It is relatively easy to achieve these strengths with the low-alloy steels of conventional quench and temper type, but they do usually have to contain sufficient carbon (>0.4 per cent C), making welding difficult, but quite practical with knowledge and experience. Typical of these steels is EN 24/AISI 4340, widely used in demanding power train applications. For the upper reaches of this category, resort is made to the precipitation hardening alloys, with higher Ni and additions of Cr and Co, making for ­expensive material.

A familiar high strength stainless steel that has been used in industry for many years is the 17-4 precipitation hardening (17.4 PH) type. These can achieve strengths in the upper range through an ageing treatment and with use of very low carbon (<0.02 per cent C), enabling the retention of good ductility for such a high strength material. There is a substantial family of alloys around this composition, as they offer additional corrosion and heat resistance, below the ageing temperatures, typically ~500°C.

The most highly developed titanium alloys, like the long established Ti-6Al-4V, can be extended well past the mid point of this category and so find significant applications in the entry section of jet engines for turbine blades where the temperature is limited. They possess a very high strength-to-weight ratio and high resistance to oxidation in these very critical applications. However, their cost and lack of lower grade uses usually makes their presence rare in the home workshop.

Normally, a more practical solution is the specialized steels, which can offer surprisingly accessible specific strengths, such as the high performance tubing met in the previous category, as Reynolds 931. This is a precipitation hardening stainless steel of the 17-4PH family, weldable for cycle frames, and providing a yield of ~1000 N/mm2 (150 ksi).

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