Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

With the exception of stress-relieving, tempering, and aging, most heat treatments begin by heating an alloy beyond the upper transformation (A3) temperature. The alloy will usually be held at this temperature long enough for the heat to completely penetrate the alloy, thereby bringing it into a complete solid solution. Since a smaller grain size usually enhances mechanical properties, such as toughness, shear strength and tensile strength, these metals are often heated to a temperature that is just above the upper critical temperature, in order to prevent the grains of solution from growing too large. For instance, when steel is heated above the upper critical temperature, small grains of austenite form. These grow larger as temperature is increased. When cooled very quickly, during a martensite transformation, the austenite grain size directly affects the martensitic grain size. Larger grains have large grain-boundaries, which serve as weak spots in the structure. The grain size is usually controlled to reduce the probability of breakage.[13]

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Some techniques allow different areas of a single object to receive different heat treatments. This is called differential hardening. It is common in high quality knives and swords. The Chinese jian is one of the earliest known examples of this, and the Japanese katana may be the most widely known. The Nepalese Khukuri is another example. This technique uses an insulating layer, like layers of clay, to cover the areas that are to remain soft. The areas to be hardened are left exposed, allowing only certain parts of the steel to fully harden when quenched.

For case hardened parts the specification should have a tolerance of at least 0.005 in (0.13 mm). If the part is to be ground after heat treatment, the case depth is assumed to be after grinding.[28]

This presentation gives a brief introduction to chemical heat treatment of steels and surface hardening techniquesKeywords: Carburising, Nitriding, Carbonitriding, Flame hardening, Laser hardening, Induction hardeningRead less

The present study evaluates the influences of PWHT on FCG behavior and tensile properties of TIG butt welded Al 6013-T4 sheets. Crack propagation tests were carried out on compact tension (CT) specimens. The T82 heat treatment was varied in three artificial aging times (soaking) of 6, 18 and 24 hours. The results of T82 heat treatment with artificial aging variations were tested for their fatigue crack growth rates at the main metal zone, the heat-affected zone (HAZ), and the welded metal zone. It has been observed that the various agings in heat treatment T82 are sensitive to the mechanical properties (fatigue crack growth rate test, tensile test). The results show that PWHT-T82 for 18 hours aging is the highest fatigue resistance, while the aging 18 hours provided the highest tensile test result.

The present work is associated with research concentrating on the innovation and use of hybrid heat treatment for eutectoid steel powder (0.8 wt%) reinforced Al-Zn-Mg (Al 7075) alloy composites. Due to high hardness, wear resistance, tensile strength and flexibility modifying heat treatment, heat treatable aluminium metal matrix composites reinforced with heat treatable hard steel particles may be the choice to explore. In this work, an attempt is made to increase hardness and tensile properties to higher levels through hybrid heat treatment, comprising simultaneous treatment to matrix and reinforcement in 3 different routes (Pearlite, Bainite and Martensite) compared to conventional age hardening treatment. SEM images and microhardness distribution witnessed the phase transformation in both matrix and reinforcement. Aging kinetics in conventional age hardening and hybrid treatments is accelerated by the increase in the quantity of reinforcement and increase in aging temperature. The improvement in hardness and tensile strength obtained by conventional age hardening path is further improved by the hybridisation path. Hybridisation route with martensite reinforcement phase shows excellent result in hardness, strength followed by bainite and pearlite path respectively in the decreasing order. In all heat treatment cycles, lower aging temperature marks greater property enhancement compared to higher temperature. Al 7075 with 6 wt% steel powder reinforced (martensite form) composite showed excellent peak age hardness, tensile strength compared to lesser quantity reinforced composites.

The age hardenable matrix can be reinforced with heat trea steel powder by stir casting method and matrix definitely subjected to age hardening treatment for property alterations without reinforcement undergoing treatment. At the same time, if reinforcement (steel powder) also undergoes heat treatment simultaneously, the heat treatment has different effect on matrix and reinforcement in a single process may be called as HYBRID heat treatment. In this work, an attempt is made to hybrid heat treatment process by combining age hardening treatment to matrix material and conventional treatments (annealing, normalizing, hardening, austempering etc.) to steel powder at a stretch. Conventional treatments yield different micro constituents (phases) in reinforcement like coarse pearlite, fine pearlite, martensite, bainite etc. with different packing factors of the microconstituents depending on the type of treatments1313 Mahathaninwonga N T, Plookphola J, Wannasin J, Wisutmethangoon S. T6 heat treatment of rheocasting 7075 Al alloy. Mater Sci Eng. 2012;532:91-9.,1414 Mandal D, Ghosh M, Pal J, De PK, Chowdhury SG, Das SK, et al. Effect of austempering treatment on microstructure and mechanical properties of high-Si steel. J Mater Sci. 2009;44(4):1069-75.. Phase transformation of reinforcement, particle experiences tensile strain within it due to lower packing factor of the transformed microconstituent phases, matrix experiences compressive strain and the resultant is reflected as property alteration, especially, hardness and UTS. At the same time matrix undergoes age hardening treatment and induces tensile strain in the matrix owing to the controlled precipitation of intermetallics (strengthening phases) during heat treatment hybridization. The combined effect of these two processes develops thermal misfit strain between the reinforcement and matrix1515 Uvaraja VC, Natarajan N, Sivakumar K, Jegadheeshwaran S, Sudhakar S. Tribological behavior of heat-treated Al 7075 metal matrix composites. Indian J Eng Mater Sci. 2015;22(1):51-61..

The purpose of the study is to uplift hardness related properties to superior levels through hybrid heat treatment, comprising combined treatment to matrix and reinforcement through novel routes (Pearlite, Bainite and Martensite) compared to conventional age hardening treatment. Even though a lot of literature reports are available on non-heat treatable reinforcement reinforced composites, heat treatable reinforcements like, steel powder and alteration in the heat treatment path to have positive impact on both matrix and reinforcement is not yet performed. In this study, such an attempt is made to obtain the composite with superior combination of mechanical properties by newly designed heat treatment procedure. In the area of age hardening composites, this work is the novel approach comprising impact of hybrid heat treatment simultaneously to reinforcement and matrix in the path of property alterations.

The methodology adopted in carrying out stir casting for manufacturing of composites is described by the flowchart shown in Figure 1. Composites are fabricated by stir casting technique. Initially, billets of Al 7075 are cut into pieces, placed in a graphite crucible and heated to melt. Melting is continued by increasing the temperature to 750C and soaked isothermally for sufficient time till uniform liquid melt results. The lower melting impurities are removed as slag by introducing small quantity of scum powder. Dry Hexa chloroethane (C2Cl6), 0.3 wt% is added to the melt for degasification. Wettability of the reinforcement is enhanced by adding pieces of Mg (1 wt%) to the melt. Mg also helps to strengthen the matrix by the precipitation of secondary intermetallic phases during the controlled heat treatment1616 Reda Y, Abdel-Karim R, Elmahallawi I. Improvements in mechanical and stress corrosion cracking properties in Al 7075 via retrogression and reaging. Mater Sci Eng. 2008;485(1-2):468-75.. The eutectoid steel powder particles are preheated to 300C for 1 h. This process removes all the volatile elements and maintains the powder crispy. The melt is then allowed to cool in air up to 700C to a semi-solid state, further stirred to form the vortex by using a stirrer made of mild steel. During stirring the vortex is formed and eutectoid steel powder in varying proportion (2, 4 and 6 wt%) is transferred into the melt in each run. Optimum distribution of the eutectoid steel particles in the alloy can be obtained by maintaining a stirrer in the range of 150-200 rpm for 15 minutes.

The age hardening treatment is carried out after solution heat treatment (SHT) at 550C for a duration of 2 h followed by quenching in demineralized water at room temperature. The quenched specimens are artificially aged in the furnace at 100 and 180C for different time intervals and hardness distribution curves are plotted against isothermal aging time. From Al-Zn-Mg ternary phase diagram, as per the ternary eutectic MgZn2-(Al) -(Zn) phase formation the solutionising temperature is fixed as 550C. The harder MgZn2 intermetallic phase found to dissolve completely at 550C during solutionising and sequentially precipitated during aging to strengthen the matrix. It is reported that the samples of Al 7075 and its composites, after solution treatment at 550C, exhibits better strength and toughness property combination1818 Zou X, Yan H, Chen X. Evolution of second phases and mechanical properties of 7075 Al alloy processed by solution heat treatment. Trans Nonferrous Met Soc China. 2017;27:2146-55.. The heat treatment cycle used is shown in Figure 2. 589ccfa754

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