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Hardenability:all you should know about.SOLUTIONS with Effective, Practical Advice
Hardenability is a definite steel property describing the depth and distribution of hardness to which that steel may be hardened during quenching from the austenite temperature. It is a material property, dependent on chemical composition and grain size, but independent of the quenchant or quenching system (cooling rate). However, the structures and hardness obtained in that steel at different depth across a quenched section are a function of both hardenability and of the quenching process (severity of quench and cooling rate). The hard constituent of quenched steel is called martensite. In other words, hardenability is a measure of the ability of a given steel to be hardened by the formation of martensite as a result of a certain heat treatment. Martensite hardness depends on carbon content. The higher the carbon content up to about 0.8%, the harder the quenched steel. The steel chemical composition will affect its ability to transform to martensite for a given quenching treatment. Other less hard structures, called bainite, pearlite, and ferrite are also transformation structures derived from austenite upon cooling at slower cooling rates. Generally, most alloying elements (excluding cobalt) delay the formation of softer microstructures and allow the higher hardness structure (martensite) to form at lower temperatures or at slower cooling rates. The alloying elements inhibit the formation of the three other quench products. Hence, they allow martensite to form at slower cooling rates (like air cooling). Hardness is a different property, measuring the resistance to indentation.
As an example to clarify the behavior of two steels of different levels of the ability to harden, consider two short pieces of bar of 25 mm The first steel called 1060 has no other alloying element besides carbon. If both bars are quenched in agitated oil from the austenitic structure for both steels, existing at the temperature of about 850 °C (1560 °F), the surface hardness of both bars will be found equal, at about However upon sectioning both bars, the hardness at the center will be found:
In summary, the steel with higher hardenability hardens (or forms martensite), not only at the surface, but throughout the section of the specimen where the cooling rate is slower than on the surface. The selection of more hardenable steels permits to obtain sufficient hardness where needed by using less severe quench to avoid problems with warping and cracking. The specific End-Quench or Jominy Test was developed for determining experimentally this property, with a repeatable testing procedure, and showing the results in graphic format. It is briefly described at section 9.4 of PWL#074. ASTM A255-10
In this procedure all factors affecting depth of hardening, except alloy composition, are kept constant. The determination is conducted by heating a standard specimen above the upper critical temperature, and by hanging the hot specimen in a fixture so that a stream of cold water impinges on the bottom end at specified flow rate and temperature. Therefore the cooling rate and the hardness are at their maximum at the quenched end, and decrease with distance from the quenched end. After the specimen has cooled down to room temperature, a shallow longitudinal flat is ground parallel to the specimen axis, and Rockwell hardness readings are taken every sixteenth of an inch along the ground flat. The data are normally plotted in a graph as Rockwell C hardness versus distance from the quenched end. A highly hardenable steel will retain high hardness values for relatively long distances, while the hardness of a low hardenable steel will drop rapidly. Each steel has its own unique hardenability curve that can be characterized by specifying only two hardness values at specified distances. Another measure of steel hardenability is given by the Ideal Diameter. It is defined as the diameter of a bar which would contain 50% martensite at its center following a quench in an ideal medium. The larger the ideal diameter, the higher the hardenability of the steel. The ideal diameter of a plain carbon steel with carbon content of 0.4% (1040 steel) and whose ASTM grain size number is 7 is The ideal diameter for a 4340 steel (0.8 Cr, 1.75 Ni, 0.25 Mo) is over Naturally, varying the grain size or changing the concentration of alloying elements (by taking a different steel) the ideal diameter will change. An empirical method of accounting for these effects would utilize a series of multiplying correction factors, making this method possibly less practical than that using Jominy curves. When considering the possible consequences of welding on the specific steel used, this property should be taken into account for designing suitable welding procedures. For an enlarged exposition of this subject, An Article on Filler Metls and shielding gas influence to avoid cracking in welds was publised (4) in Issue 147 of Practical Welding Letter for November 2015.
Click on the following link to view an example of Jominy Test results on a few different steels. Check the hardness at depth. Hardenability Curves for different Grade of Steels.
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