Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process. The result is a structure of rods or spheres of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel. The image to the right shows where spheroidizing usually occurs.
The addition of boron to steel allows the achievement of higher strength after hardening by heat treatment, but offers a workable material to the fabricator or manufacturer when in the “as-delivered” condition.
Boron is often added to medium carbon steels to achieve an in-service performance comparable to high carbon and more costly low alloy steels (rather than increasing their carbon and manganese content or adding chromium and molybdenum – with the attendant penalty of reduced ductility during fabrication).
The amount of boron which is added to achieve these characteristics is very small – in the range 0.0005-0.005%.
Traditional applications for boron steels are in wear applications such as shovels/spades, caterpillar tracks, plough shares, punches, but also some spring steels, and more recently in automotive car bodies. Here they have been developed into high strength sheet steels for parts of the body shell and chassis – such as door sills, door pillar reinforcement, cross members, safety beams and bumper reinforcements.
When the Brinell test is used on very hard materials, low values result owing to the spherical shape of the indenter and flattening of the ball. These are eliminated by using a square based diamond pyramid indenter which does not deform easily and gives geometrically similar impressions under various loads. The diamond pyramid with an angle between opposite faces of 136 degree is pressed under a standard load into the surface of the material, and the diagonal of the indentation produced is measured- The load divided by the contact area of impression gives the Vickers Pyramid Number. VPN = Load in kg / Pyramidal area of impression in sq. mm