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Heat Treating ARE YOU GETTING WHAT YOU PAY FOR? If you buy sprockets or gears with hardened teeth you should know that not all hardening is created equal. In many cases, parts sold as having hardened teeth are sub-standard in terms of what we consider to be good heat treating. Fundamentally, heat treating is a process which changes the microstructure of steel in order to achieve an even distribution of carbide crystals throughout the base structure. Carbide starts out in "large globs" which are diffused into smaller crystal groupings when the part is heated to a high temperature. Quenching in fluid immediately after heating causes a very rapid reduction of temperature which "freezes" the carbide crystals in a well distributed condition. After quenching, heat treated parts should be tempered (reheated to a lower temperature and slow cooled) to increase ductility (reduce brittleness) by lowering the post-quench hardness a few points. The resulting structure of small, evenly distributed grains gives heat treated steel its hardness. There are three processes which are used to do the majority of sprocket and gear hardening - flame, induction and carburizing. All of these are good processes when done properly. The first, flame hardening requires the least investment in equipment and training. In fact, it can be done, in an unsophisticated manner, with tools found in most any work shop. A part is simply heated with any gas torch until it is red hot, then it is quenched, usually in water. This process is difficult to do well and is probably the easiest to do poorly. It is also one of the most common methods for hardening sprockets and gears. What we commonly see with parts flame hardened by non-professionals is inconsistency. Often we find very hard areas and very soft areas within a single part. Sometimes only the very tips of the teeth are actually hardened properly, or there is insufficient depth of hardness. Coarse pitch gears sometimes have major variations of hardness across the face of a single tooth. Multiple strand sprockets are often hard on one end and get softer as you progress across the strands. We even see cases where all that was achieved by flame "hardening" was a discoloration of the part without any hardening result. The old "buyer beware" motto seems to be appropriate in many cases. The only way to know that you are getting what you pay for is to buy hardened sprockets and gears from sources which have the ability to do the job right, or have the parts you buy tested with the proper specialized testing devices, either in your own plant or a third party testing lab. Such testing usually comes in the form of checking the hardness of a given part at various points in the area to have been hardened and comparing them with targets set prior to heat treating. Hardness can be rated on several different scales. The Brinnell and Rockwell scales are very common in the U.S. As a point of reference, 1035 hot rolled and annealed steel usually measures about 5 - 15 on the Rockwell C Scale. Gears are typically hardened anywhere from Rockwell C 30 to 65, depending on their type, size, material, application and hardening method. We harden sprockets to R/C 40 - 50. In addition to testing parts for their actual hardness, parts must also be checked for cracking using a spray penetrant or magnetic particle method. The other two heat treating processes, induction and carburizing, are more complex methods requiring large investments in equipment and personnel. As a result they are usually seen only in professional heat treating environments such as you would find in a commercial heat treater's shop or a manufacturer's dedicated shop such as our own induction facility. In this environment the results of heat treating processes can be controlled by qualified people using the right equipment to achieve the desired results. cont. |