November/December AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 8 6 1 material to be carburized, and the required case depth and temperature accuracy shall conformto AMS2750Ewith ±10°F furnace temperature uniformity for Class 2 type furnaces. Gas carburizing calls for temperatures between 1500° and 1750°F. Reasons for poor furnace design include inadequate heating systems or heating zone shapes, poor furnace in- sulation integrity, and gas circulation systems that are too small or incorrectly designed. A heating space with free and unobstructed atmosphere circulation along with a proper recuperative heating system for gas or electric heating ele- ments ensures uniform heating. The heating rate should be controlled to avoid damage to parts. AMS2759/7B recom- mends parts with complicated shapes to be subcritically an- nealed or preheated prior to austenitizing to avoid cracking and minimizing distortion. Recuperative burners increase thermal efficiency up to 75% by utilizing the heat from ex- haust gases to preheat the combustion air and improve tem- perature uniformity. This thermal efficiency allows better control of tem- perature uniformity and heat distribution within the heating space. These burners are paired with single-ended radiant tubes that are fitted with special ceramic inner tubes, which offer additional benefits of low noise, high durability, and easy installation. This allows them to provide ideal heating while optimizing gas consumption and lowering the cost of operation. For many low alloy steels and steel with large cross sections, liquid quenching is required. Oil or polymer solu- tions may be used as specified for the alloy indicated. The consistency of quench effectiveness is determined by the quench tank design and quenching controllability during the process. Each tank should be tested initially and quar- terly thereafter by one of themethods specified by AMS2759. The heat treating facility should establish control limits for each quenching system. If results indicate that a quenchant is outside of established limits, corrective action should be taken and the tests repeated to verify restoration of the prior condition. Variable speed converters regulate oil circulation for uniform and proper speed depending on temperature, material, and part shape. Variable speed oil circulation sys- tems reduce the hardness spread and increase overall fur- nace throughput. The ability to control the flow of oil allows, to some extent, equalization of the core and surface tem- perature. By doing so, this controls distortion during critical phase transformations. DISTORTION DUE TO STRUCTURAL CHANGES Metal expands as the temperature increases until it reaches transformation temperature (Ac1). From tempera- ture Ac1, at which austenite begins to form during heating, to temperature Ac3, at which transformationof ferrite to aus- tenite is completed, the metal “shrinks.” After the tempera- ture equalizes and transformation ends, the metal begins to expand again, causing internal stress. If the internal stress exceeds the yield point, distortion occurs. Expansion and shrinking may occur during heating and cooling. All changes except the formation of martensite occur at lower tempera- tures during cooling rather than during heating, and depend on the rate of change in temperature. To control this type of distortion, ensure a uniform tem- perature throughout the part during transformation. The actual time of equalization is determined by the size and complexity of the part. DISTORTION DUE TO RESIDUAL STRESSES Machining, forming, or welding before heat treatment will cause residual stresses in the part. The heating of one 9 Fig. 2 — Two types of distortion. TABLE 1 — MODULUS OF ELASTICITY AND THERMAL CONDUCTIVITY Modulus of Elasticity Coefficient of Expansion Thermal Conductivity Metal GPa psi × 10 6 10 -6 /K 10 -6 /°F W m -1 k -1 Btu in./ft 2 • h • °F Pure iron (ferrite) 206 30 12 7 80 555 Typical austenitic steel 200 29 18 10 15 100 Aluminum 71 10 23 13 201 1400 Copper 117 17 17 9 385 2670 Titanium 125 18 9 5 23 160