Heat Treatment, Selection, And Application Of T...
Safety is a prime consideration when determining the type of heat-treating process necessary for a project. Deciding whether to use heat-strengthened or tempered glass depends on the specific application. For example, heat-strengthened glass can be selected for applications that do not specifically require a safety glass product; and tempered glass should be used wherever safety glass is a requirement.
Heat Treatment, Selection, and Application of T...
High strength steels may follow an industry standard classification such as ASTM, ABS, AISI; or they may be proprietary to a steel manufacturer, in which case they will not fit traditional classification systems. For proprietary high strength alloys, the steel manufacturer can provide valuable insights into welding procedures, filler metal recommendations, and pre- and post-weld heat treatment, as well as recommended interpass temperature controls.
To begin the heat sink selection, the first step is to determine the heat sink thermal resistance required to satisfy the thermal criteria of the component. By rearranging the previous equation, the heat sink resistance can be easily obtained as
With all the parameters on the right side of the Rsa expression identified, it becomes the required maximum thermal resistance of a heat sink for the application. In other words, the thermal resistance value ofa chosen heat sink for the application has to be equal to or less than Rsa value for the junction temperature to be maintained at or below the specified Tj.
When selecting a heat sink, it is necessary to classify the air flow as natural, low flow mixed, or high flow forced convection. Natural convection occurs when there is no externally induced flow and heat transfer relies solely on the free buoyant flow of air surrounding the heat sink. Forced convection occurs when the flow of air is induced by mechanical means, usually a fan or blower. There is no clear distinction on the flow velocity that separates the mixed and forced flow regimes. It is generally accepted in applications that the effect of buoyant force on the overall heat transfer diminishes to negligible level (under 5%) when the induced air flow velocity excess 1 2 m/s(200 to 400 lfm).
The above tabulated ranges assume that the design has been optimized for a given flow condition. Although there are many parameters to be considered in optimizing a heat sink, one of the most critical parameters is the fin density. In a planar fin heat sink, optimum fin spacing is strongly related to two parameters: flow velocity and fin length in the direction of the flow. Table 3may be used as a guide for determining the optimum fin spacing of a planar fin heat sink in a typical applications.
One can use the performance graphs to identify the heat sink and, for forced convection applications, to determine the minimum flow velocity that satisfy the thermal requirements. If the required thermal resistance in a force convection application is 8 C/W, for example, the above sample thermal resistance versus flow velocity curve indicates that the velocity needs to be at or greater than 2.4 m/s (470 lfm). For natural convection applications, the required thermal resistance Rsa can be multiplied by Q to yield the maximum allowable Tsa. The temperature rise of a chosen heat sink must be equal to or less than the maximum allowable Tsa at the same Q.
As noted in Table 1, the two methods each have advantages and challenges [1,2]. Chemical transformation or heat shock can be performed in a simple lab setup, yielding transformation efficiencies that are usually sufficient for routine cloning and subcloning applications. Since the cell membrane is made more permeable by cation treatment and heat shock, certain cell types, such as those with cell walls, may not be favorable to chemical transformation.
In contrast, heat shock of chemically competent cells offers a more flexible setup for different throughputs (Figure 5). For low-throughput experiments, cells in individual tubes may be transformed with DNA by heat-shocking directly and conveniently, ensuring no loss of transformation efficiency from repeated freeze/thaw cycles. A similar approach may be considered for medium-throughput cloning, using a multichannel pipettor and cells in strip tubes. For high-throughput applications, 96-well formats allow multichannel pipetting, block incubation, and even automation, if needed.
By way of definition, heat resistant applications typically occur above 1200F/670C and require the use of materials that have enhanced resistance to oxidation and other environment-specific gases and mechanical property degradation. Performance in these high-temperature environments is indicated by acceptable levels of tensile strength, stress rupture life, and creep strengths that correspond to the required time of service.
In the case of high strength requirements at elevated temperature, cyclical thermal exposure or aggressive carbonaceous atmosphere (and carbon is the enemy in certain high-temperature applications like petrochemical furnaces), nickel based alloys are typically selected. However, cobalt-based alloys may also be used. The primary tradeoff is usually economic. Comparing a high initial cost versus the life cycle cost of a conventional heat resistant alloy will help to determine the best long term value.
High temperature applications demanding heat resistant materials occur frequently in industry. Those applications include power plants, mineral pyro processing (cement, lime, and iron ores, for example), waste incineration, petrochemical processing, steel and non-ferrous mills, metal processing including heat treating, and glass making / forming.
In discussing applications of heat resistant castings, there are obvious tradeoffs between the life cycle cost of more expensive proprietary alloys and the more conventional alloys that may be encountered in the field. It may be helpful to classify the alloys using five frequently used categories. The following introduction provides some perspective and a general framework that may be used to classify alloys being considered for an application.
Creep is the strain, defined per unit of time, that occurs under stress at elevated temperatures. Creep occurs in many applications of heat-resistant castings at service temperatures. Over time, creep may lead to excessive deformation, which may further lead to fracture at stresses well below those that would cause a fracture in a tensile test at the same temperature.
Induction coil design has a major impact on process efficiency and final part quality, and the best coil design for your product largely depends on your application. Certain coil designs tend to work best with specific applications, and a less than optimal coil-application pairing can result in slow or irregular heating, higher defect rates, and lower quality products.
Part motion relative to coil - Several applications rely on part movement with the help of conveyors, turntables, or robots. A properly designed induction coil incorporates these individual handling requirements without the loss of heating efficiency.
Frequency - Higher frequencies are used for applications like brazing, soldering, annealing or heat treating, where surface heating is desired. Lower frequencies are preferred for applications requiring through-heating of the parts to the core like forging and die heating.
Powder-density requirements - Higher power densities are required for short cycle heating applications requiring high temperatures. Higher power densities may also be required to keep the hot zone confined to a small area, reducing the heat affected area.
Three common rack materials for heat-treat applications where temperatures exceed 815C (1500F) are heat-resistant steel alloys, molybdenum alloys and carbon-carbon composites. Table 1 displays a variety of relevant properties for these three materials.
In some cases, a metal part may go through several heat treatment procedures. For instance, some superalloys used in the aircraft manufacturing industry may undergo up to six different heat treating steps to optimise them for the application.
Heat treating (or heat treatment) is a group of industrial, thermal 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 the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although 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.
Untempered martensitic steel, while very hard, is too brittle to be useful for most applications. A method for alleviating this problem is called tempering. Most applications require that quenched parts be tempered. Tempering consists of heating steel below the lower critical temperature, (often from 400F to 1105F or 205C to 595C, depending on the desired results), to impart some toughness. Higher tempering temperatures (maybe up to 1,300F or 700C, depending on the alloy and application) are sometimes used to impart further ductility, although some yield strength is lost.
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.[30]
Salt baths utilize a variety of salts for heat treatment, with cyanide salts being the most extensively used. Concerns about associated occupation health and safety, and expensive waste management and disposal due to their environmental effects have made the use of salt baths less attractive in recent years. Consequently, many salt baths are being replaced by more environmentally friendly fluidized bed furnaces.[33] 041b061a72