Instead a structure called martensite is formed This is the hardest structure

Instead a structure called martensite is formed this

This preview shows page 36 - 38 out of 73 pages.

Instead, a structure called martensite is formed. This is the hardest structure that is it is possible to produce in a plain carbon steel and, under a microscope, it appears as acicular (needle-shaped) crystals as shown in Fig. 3. Actually, these are sections through disc-shaped plates. What has happened is that the face-centred crystals of austenite have changed to body-centred crystals below the Ar 1 line as usual but, because of the rapid cooling, there has not been time for the cementite to form and the body-centred crystals are a supersaturated solution of carbon in iron (martensite). This so distorts the lattice structure that slip virtually becomes impossible and the steel becomes very hard and brittle. Large components do not cool quickly as small components and may not achieve the critical cooling rate necessary for maximum hardness. The critical cooling rate is defined as the slowest cooling rate (quenching rate) which will produce a martensitic structure throughout the mass of steel. If this cooling rate is not achieved some pearlite will be formed and the steel will be tougher but substantially less hard. However, there is no virtue in exceeding the critical cooling rate to any extent. Once maximum hardness has been achieved any increase in the cooling rate will only result in cracking and distortion of the workpiece. Further, there is no particular advantage in heating hyper-eutectoid steels above their Ar cm temperature when hardening them and, in practice, the hardening temperature for hyper-eutectoid steels is just the Ar 1 temperature (see Fig. 1). Quenching hyper-eutectoid steels from this lower temperature helps to prevent grain growth, cracking and distortion.
Image of page 36
~ Page 37 of 73 ~ The critical cooling rate can be substantially reduced by the addition of alloying elements to the steel. This enables thicker components to be hardened with less chance of cracking and distortion and is one of the most important reasons for using alloy steels. QUENCHING MEDIA The most commonly used quenching media in order of severity are: o Compressed air blast – least severe. o Oil. o Water. o Brine (10% solution) – most severe. The choice of quenching bath depends upon the type of steel being treated and the resultant properties required. Brine, which is a solution of common salt and water, is only occasionally used to provide very rapid cooling for plain carbon tool steels and case hardening steels where maximum hardness is required. Such severe quenching can lead to cracking in all but the simplest components, and plain water and quenching oils are most commonly used for plain carbon and alloy steels. To avoid cracking and distortion the quenching rate should be no greater than that needed to give the required properties in the workpiece.
Image of page 37
Image of page 38

You've reached the end of your free preview.

Want to read all 73 pages?

  • Fall '19

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask You can ask (will expire )
Answers in as fast as 15 minutes
A+ icon
Ask Expert Tutors