Tempering is used to improve toughness in steel that has been through hardened by heating it to form austenite and then quenching it to form martensite.
At these temperatures the martensite decomposes to form iron carbide particles. The higher the temperature, the faster the decomposition for any given period of time. The micrograph shows a steel after substantial tempering. The black particles are iron carbide. Untempered martensite is a strong, hard, brittle material. The stronger and harder it is, the more brittle it is. The strength and hardness is a due to elastic strain within the martensite, which is a result of too many carbon atoms being in the spaces between the iron atoms in the martensite.
As the amount of carbon in a steel increases up to about 0. Need help selecting a steel alloy or heat treating process? The professionals at APP will be glad to answer any technical questions about industrial pipe designs and materials. Hi, there! Need a quick quote or have a pipe support question? Complete this form and an APP representative will be in touch with your quote. Home blog Carbon Steel vs. Carbon Steel vs.
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Recent Posts. Contact Us Headquarters Guhn Rd. A steel's grain size depends on the austenitizing temperature. When a steel that will transform is heated to slightly above its A 3 temperature and then cooled to room temperature, grain refinement takes place. Fine grain size offers better toughness and ductility.
Austenitizing temperatures higher than 1, degrees F generally cause a coarse austenitic grain structure, and these coarse-grained steels are usually inferior to fine-grained steels in terms of strength, ductility, and toughness. Steel forgings and castings are often normalized specifically to refine their grain structure. It should be apparent that it sometimes requires a lot to strengthen a metal correctly.
So, just how much do you affect all that work when you weld two pieces of hardened steel together? It depends. First, recognize that it's not just the welded joint but the entire heat-affected zone HAZ that's subject to influences from welding heat. Defined as the portion of the base metal whose mechanical properties or microstructure have been altered by the heat of welding, brazing, soldering, or thermal cutting, the HAZ can sometimes be quite large.
Second, it depends on what form of strengthening was used. For example, work-hardened metals recrystallize and soften substantially in the HAZ. Solid solution-hardened metals will have a little grain growth next to the fusion line, but it's usually only a few grains wide and has little effect on the metal's properties.
Transformation-hardened alloys with enough hardenability to form martensite or have formed martensite during previous heat treatment react much like a solid solution-hardened metal: There's little change in the HAZ compared to other hardening techniques, aside from minor grain growth at the fusion line.
Precipitation-hardened metals go through some complex changes, but the result is similar to work-hardened metals: Base metal in the HAZ goes through an annealing cycle and softens. That covers the basics of metal strengthening techniques and how those techniques influence a metal's microstructure.
Next time we'll take a closer look at how welding affects hardened metals and what you can do to reduce those effects, and then we'll get into welding metallurgy. Email Bob Capudean. Read more from this issue. View the Digital Edition. This magazine has served the welding community in North America well for more than 20 years. Enjoy full access to the digital edition of STAMPING Journal , which serves the metal stamping market with the latest technology advancements, best practices, and industry news.
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