Modifying the Substrate without Altering the Substrate's Chemical Constitution

i) By heating

When dealing with transformation hardenable alloys, in particular carbon steels, low alloy steels and cast irons, the option to harden using flame, induction, laser or electron beam techniques may be the most attractive. In this case, instead of heating the whole component (as in through hardening), only the surface is affected, so that the bulk properties, specifically the toughness, remain unaffected, and component distortion is minimised.

These processes can be fully automated and precisely controlled. The desired core properties can be developed by standard heat treatment practices and the surfaces hardened by rapidly heating them to approx 850°C and then quenching. In most cases, it is prudent to follow the hardening cycle by a low temperature treatment to relieve the internal stresses (tempering).

• Induction Hardening is achieved via surface heating from a purpose designed water-cooled induction coil. Hardening depths of several mm are usual and the process is amenable to accurate control and automation, ideally for large numbers of identical components.

• Flame hardening is achieved through the local application of an oxyacetylene flame (usually by hand) so that the process is less well controlled. However, it is ideal for treating specific areas (those needing wear resistance) of complex- shaped components.

• Laser hardening can now compete in high volume production with other low cost processes such as induction hardening. Using self-quenching techniques it is possible to obtain case depths of 0.75mm. Lasers are particularly useful for hardening relatively small or inaccessible areas.

• Electron beam techniques have similar attributes and may be more economical because both the capital and operating costs are lower. The beam operates in a vacuum but the workpiece need only be at 60mbar pressure. Area hardening is obtained by scanning the area on a rastor.

ii) By mechanical working

Cold working the surface by peening, shot blasting or other specialised machining processes to produce deformed layers increases the stored energy and compressive stress, thereby increasing the hardness, fatigue and stress corrosion resistance.

In particular, shot peening has developed into a sophisticated process, with automation, computerised control, and highly reproducible properties. It imparts a compressive load into the surface, effectively increasing the tensile strength. As each individual shot particle strikes the metal surface it produces a concave depression, with plastic flow and radial stretching of the surface around the contact. In a part completely covered with shot impressions, the residual compressive stress layer usually extends to about 0.13 to 0.15 mm below the surface. Below that depth, a tensile stress layer develops to achieve an equilibrium.

The benefits obtained are the result of the effect of the compressive stress and the cold work that is induced in the surface, with increased resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen-assisted cracking, fretting, galling and erosion caused by cavitation. Additionally, the surface cold working increases the hardness, helps resist intergranular corrosion, provides surface texturing, and can close up surface porosity in coating.

It is often used as a precursor to other surface engineering techniques which might otherwise impair the fatigue or mechanical performance of a component

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