High hardenability steels

The most important application of chromium is in the production of steel. High-carbon and other grades of ferro-chomium alloys are added to steel to improve mechanical properties, increase hardening, and enhance corrosion resistance. Chromium also is added to cobalt and nickel-base alloys for the same purpose. 

The core of the bullet can be made from a variety of materials lead is by far the most common because of its high density and the fact that it is cheap, readily obtained, and easy to fabricate. But copper, brass, bronze, aluminum, steel (sometimes hardened by heat treatment), depleted uranium, zinc, iron, tungsten, rubber, and various plastics may also be encountered. (When most of the fissile radioactive isotopes of uranium are removed from natural uranium, the residue is called depleted uranium. Depleted uranium is 67% denser than lead, and it is an ideal bullet material and is very effective in an armor-piercing role, both in small arms and larger munitions components. Because of its residual radioactivity its use is controversial.) Bullets with a lead core and a copper alloy jacket are by far the most common. 

The martensitic stainless steels are iron-chromium alloys with greater than 10.5% chromium which can be hardened by suitable cooling to room temperature following a high-temperature heat treatment. Because of the low chromium content and high carbon content the corrosion resistance of martensitic stainless steels is limited compared with other stainless steels. On the other hand martensitic stainless steels are hardenable and exhibit high strength and hardness. These steels are relatively low-cost alloys. 

Most martensitic steels are hardened by heat treatment. These steels tempered below 425°C have good corrosion resistance. When the steels are tempered at about 425-540°C, resistance to SCC and hydrogen embrittlement is increased. The corrosion resistance of the steels in potable water and high-purity water at high temperature is satisfactory. These steels are highly resistant to flow erosion and erosion-corrosion. These steels are useful in applications such as boat propellers, propeller shafts and pump impellers. 

Whether decarburization will be an issue for internal combustion engines burning H2 is difficult to predict from existing information. Low-alloy carbon steels begin to decarburize at temperatures around the operating temperature of exhaust valves, but exhaust valves and valve seats are made from high-alloy steels, austenitic alloys, and superalloys where the carbon is much more stable than low-alloy carbon steels. The hardenable martensitic valve stems of exhaust valves may experience decarburization over extended periods, and this would lead to accelerated wear because of the softened surface that results from decarburization. 

Austenite has a mnch higher solubility for carbon than other forms of steel. Heating the steel to an anstenitizing temperatnre canses any carbides present to dissolve. Alloys capable of forming anstenite at high temperatnres, bnt that transform to other crystal structures at lower temperatures, are said to be hardenable by heat treatment. Martensitic steels are an example. Most carbon and low-alloy steels are hardenable by heat treatment. 

This method is particularly suitable for steels of higher carbc content showing high hardenability, where it is required to prodiv a softer HAZ microstructure than martensite directly after weldii and without recourse to postweld tempering. 

Other elements are present, but in amounts such that the CE does not greatly exceed 0.60, i.e. insufficient to confer high hardenability in this group, hardenability is high if welding a combined thickness of 25 mm at a heat input of 1.4kJ/mm fully hardens the HAZ. The group is the third major group of weldable steels. 

Three methods of establishing welding procedures for highly hardenable steels were described in Chapter 2. These involved the use of ... 

Moulds must resist high pressures without distortion, and resist wear over 10 cycles or more they are usually made from forged blocks of low-alloy steel, air-hardened after machining. Moulds act as thick-walled pressure vessels, with high tensile stresses in the walls. For a melt pressure of 50 MPa and a cavity diameter that is four times the wall thickness, the average hoop stress in the wall is 100 MPa from Eq. (C.21). Concave corners in the mould cavity act as stress concentrating features, so to... 

The one-impression mould was made by Penton Tools Ltd of High Wycombe (part of the Guinness group). It is a conventional two-plate injection mould incorporating a side core to produce the front sliding cover channel. The mould was made from mild steel with hardened-steel cavity inserts. 

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