A discussion of diffusion and its effects can be found in a set of lectures available online. Steel is incredibly versatile because it can be heat-treated in order to produce a vast range of microstructures and associated mechanical properties. The heat-treatment usually involves the steel being heated into a temperature in the austenite phase field. This temperature is quite high, in the range o C, depending on the details of the chemical composition.
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One such risk is decarburization, which can weaken steel parts and put them at greater risk of failure during their service. Preventing decarburization in steel requires metallurgists and their customers to pay attention to the details. Decarburization occurs during the interaction between the carbon atoms in steel and the atmosphere of an endothermic atmosphere furnace.
It happens when carbon content inside the furnace is not properly controlled. Decarburization can occur during manufacturing prior to heat treatment and also as a result of heat treatment processes. Parts where decarburization is often observed prior to heat treatment include investment casting parts, forgings and castings and medium-carbon steels derived from hot-rolled barstock. Investment castings can have significant decarburization stemming from sub-par conditions in foundry equipment, but this can be reversed during heat treatment with a carbon restoration.
For forgings, castings and barstock, decarburization can either be machined off or fixed via carbon restoration. The surface of a part is most affected during decarburization.
Because carbon is drawn out through the surface, the result is a part with less carbon content near the surface compared to the core. With less carbon at the surface, a part becomes less resistant to stress.
Parts that encounter extreme rotating or alternating forces —structural bolts, for example— are prone to fail if decarburization occurs. Decarburization also presents added quench cracking risks for parts made of more hardenable alloys. Usually, decarburization during heat treatment occurs when carbon is not adequately controlled in the furnace. For example, the depth and extent of decarburization can be determined by using a Knoop microhardness test. Hardness is measured at greater and greater depths perpendicular to the part surface until a constant hardness is observed.
Decarburization depth measurement can also be conducted with metallographs. Metallurgists use these powerful microscopes to observe and measure how deep from the surface carbon loss has occurred.
In some cases, the damage done by decarburization can be undone via a carbon restore. This involves putting a part back into the furnace with the atmosphere calibrated to replace the carbon that was previously drawn out. Investment castings and, to a lesser extent, hot-rolled barstock parts often have decarburization present prior to heat treatment. Millstock present on these parts prior to heat treatment should be machined off to help avoid decarburization.
Then, the burden shifts to heat treaters. Ensuring correct calibration and control of heat treating equipment is critical. At Paulo, we combine top-of-the-line equipment and leading expertise with a computerized process control system we built ourselves. It monitors every aspect of a job and lets our experts see furnace conditions in real time. With decades of experience on heat treatment projects of all types and sizes, you can be sure your parts are made better by the best in the business.
Contact a metallurgist if you want to know more about preventing decarburization. You can also learn more about how an outsourcing partnership can enhance heat treating risk management by downloading our free guide. Too often, metallurgists receive inadequate heat treatment specifications. Some specs contain too little information. Some are unclear.
Some are just…. One of the keys to successful manufacturing of metal parts is understanding how the journey from raw material to finished…. Heat treatment is an end-stage process often followed by delivery to customers and payment to you.
Because you like getting…. Learning Center. Measuring and preventing decarburization in steel August 29, Risk management in heat treating: Process knowledge that has your back. Download the guide. Previous post Next post. At Paulo, we love to solve problems. Keep up with our latest solutions. Or follow us online. LinkedIn Facebook. Risk management in heat treatment: Process knowledge that has your back. Related Content Understanding heat treatment specifications Too often, metallurgists receive inadequate heat treatment specifications.
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Decarburization of Steel
Decarburization or decarbonization is the process opposite to carburization , namely the reduction of carbon content. The term is typically used in metallurgy, describing the reduction of the content of carbon in metals usually steel. Decarburization can be either advantageous or detrimental, depending on the application for which the metal will be used. It is thus both something that can be done intentionally as a step in a manufacturing process, or something that happens as a side effect of a process such as rolling and must be either prevented or later reversed such as via a carburization step. The decarburization mechanism can be described as three distinct events: the reaction at the steel surface, the interstitial diffusion of carbon atoms and the dissolution of carbides within the steel.
Measuring and preventing decarburization in steel