Production Machining

JUN 2018

Production Machining - Your access to the precision machining industrial buyer.

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productionmachining.com :: 19 Helping Precision Machine Shops Be More ProducƟ ve and Profi table Technical Regulatory Management Quality Technical Regulatory Management Quality Craftsman's Cribsheet Miles Free – Director of Industry Research and Technology NUMBER 66 5 Ways Fine Austenitic Grain Size Affects a Machine Shop While austenitic grain size is a result of chemistry, the changes it evokes in our processes are a result of material structure and properties, not just the chemical ingredients. Steel that is fully deoxidized with refi ned grain is less susceptible to cracking and distorting and more easily controlled in heat treat. Using steel like this is well worth it when it comes to fi nal performance, compared with the machinist's increased tooling costs. Here are fi ve ways fi ne austenitic grain size affects your machine shop. Compared with coarse- grained steels, using fi ne austenitic grain sizes results in: 1. Poor machinability. The hard oxides and nitrides resulting from deoxidation and grain refi nement abrade the edge of tools and coatings. This is one of the reasons shops go through more tooling on fi ne-grained steels. 2. Poor plastic forming. 3. Less distortion in heat treating. 4. High ductility. 5. Shallow hardenability. Fine austenitic grain size is a result of deliberately adding grain refi ning elements to a heat of steel. Because these grain refi ning elements have been added, the steel has a fi ne austenitic grain size. In order to make steels with this austenitic fi ne grained structure, the steel is fi rst deoxidized, usually with silicon. Then aluminum, vanadium or niobium is added. Aluminum, vanadium and niobium are called grain refi ners. After the silicon has scavenged most of the oxygen out of the molten steel, the grain refi ner is added. Using aluminum as an example, the added aluminum reacts with nitrogen in the molten steel to form aluminum nitride particles. These tiny particles precipitate along the boundaries of the austenite, as well as within the austenite grains. This restricts the growth of the grains. Because the deoxidation and grain refi nement create hard, abrasive oxide and nitride particles, they machine and process differently than coarse- grained steels. Fine austenitic grain size appears on the material test report as an ASTM value of fi ve or more. Values of fi ve, six, seven, eight or "fi ve and fi ner" indicate the material is austenitic fi ne grained. Typically, seven or eight was reported for the aluminum fi ne grain steels I certifi ed. Alloy steels are always made to fi ne austenitic grain size. Most free machining steels, those that are re- sulfurized (11XX), resulfurized and rephosphorized (12L14, 1215) and leaded (12L14, 11L37, 11L44) are generally made to a coarse grain practice. Fine grain steels will not machine nor have the extended tool life we expect from the coarse- grained free machining steels we prefer to machine. The methods for determining austenitic grain size are detailed in "ASTM Standard E112, Standard Test Methods for Determining Average Grain Size." All Craftsman's Cribsheets are available for viewing and download at pmpa.org/knowledge-tools/craftsmans-cribsheets.

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