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Heat Treatment and Heat Treating Standards
The Process of Heat Treatment
Heat treatments harden steel and other metals, increasing their hardness and resistance to wear. Heat treatments may be done while the metal is immersed in a solution or simply exposed to heating elements. The depth of the hardening refers to how deep the heat treatment phase change goes into the metal piece.
The Society of Automotive Engineers or SAE, ASTM International and ISO have issued standards for the heat treatment of metals.
Leading Heat Treatment Standards
Society of Automotive Engineers or SAE standard J413 gives the mechanical properties of heat treated steel. However, not all steel alloys can be heat treated. SAE AMS-H-6875B is the standard for the heat treatment of steel parts.
SAE J437 gives the recommended process for selecting and heat treating steel used to make tools, maximizing their life. AS7479 gives the standards for bolts and screws made from heat treated iron alloys.
While steel is one of the metals most often heat treated, there are separate standards for the heat treatment of other metals. SAE AMS 2728A is the standard for the heat treatment of copper beryllium alloy.
ASTM B918-09 gives the standard for heat treating aluminum alloys. SAE AMS 2773 is the standard for heat treating nickel alloy parts. SAE/AMS 2775A is the standard for the case hardening of titanium alloys
Decarburization, Carburization and Heat Treatment
Decarburization refers to the loss of carbon content from the metal during the heat treatment process. Carburizing is the process of adding carbon to the surface of metal. SAE/AMS 2757 is the standard for gaseous nitrocarburizing, adding nitrogen into the surface of the metal. SAE AMS 2756B is the standard for the gas nitriding of steel parts. ASTM A355-89 is the specification for nitriding steel bars.
Military standard MIL-STD_1876A was the U.S. Department of Defense standard for gaseous nitriding, but this military standard has been cancelled. Nitriding can also be performed with a molten salt bath. SAE AMS 2755E is the SAE standard for using a low temperature molten salt bath. SAE AMS 2577F, also a standard for nitriding using a molten salt bath, was cancelled in 2009.
ASTM F2328-05 outlines the test procedure for determining the carburization and decarburization on steel fasteners like screws and bolts. ASTM F2328M-07 describes the method for determining both carburization and decarburization of threaded bolts and screws after they have been hardened. SAE ARP 1341B gives SAE’s recommended procedure for determining carburization and decarburization in carbon steel alloy.
Hardness and Hardenability Tests
Hardenability measures the depth and distribution of the hardness created by quenching iron alloys. ASTM A255 outlines the hardenability test methods approved by the ASTM. Hardness can be determined using standard tests like the Vickers hardness test, Rockwell hardness test or Brinell hardness test.
Standard Ratings for Hardness and Hardenability
SAE alloy grades have different hardenability ratings. Steels are grouped into low, medium and high hardenability. Quenching is the step when the heat treated steel is suddenly cooled. Safety in hardening refers to the ability of the steel to handle both very high heat and then be quenched quickly.
Metals with the highest safety in hardening are the least sensitive to heat treatment. Safety in hardening ratings range from very high, designated by the letter A, to low, represented by the letter E.
Heat Annealing Standards
Heat annealing is a form of heat treating metals or glass. The material is heated up and then cooled down slowly. Heat annealing reliefs the material stresses within the material that could cause it to fracture and crack under load and may increase the material’s toughness.
ASTM A666-10 is the material standard for annealed stainless steel strips and bars. ASTM B709-04 covers the process of annealing iron alloy sheets and plates in a solution.
ASTM C336-71, issued in 2010, is the procedure for testing the annealing point of glass using fiber elongation. Other ASTM standards cover a specific alloy when it has been processed, including the process of annealing. For example, ASTM A726-05 covers cold rolled magnetic lamination steel that has been partially processed. ASTM A726-05 allows for the metal to have been semi-processed, whether this was done through cold-rolling or annealing. The ASTM A536-84 standard for ductile iron castings allows for the castings to be heat treated or annealed.
Related Heat Treatment Standards
SAE J415 gives the standard definitions for terms related to heat treating. ISO 17663 gives the quality requirements for heat treatment in connection with welding. SAE AMS standard 2649C outlines the process for etching heat treated steel to determine if there has been overheated due to machining or grinding. Over-heating of the heat treated material can cause unwanted carburization.