Material properties generally degrade as temperature rises. This is especially visible in carbon steel. Metal companies have developed alloy steels containing nickel and chromium that, among other things, add significant improvements in strength and ductility. Historically, the alloys corresponding to ASTM A297 “H-series” cast stainless steels have been the most commonly used materials in these applications, though many proprietary alloys have gained favour in recent years.

High chrome nickel austenitic alloys, also known as heat resistant stainless steel, nickel-based alloys, cobalt chrome nickel-based alloys, and molybdenum titanium alloys are the main groups of heat-resistant alloys.

Nickel-based alloys are typically chosen for high strength requirements at elevated temperatures, cyclical thermal exposure, or aggressive carbonaceous atmospheres (and carbon is the enemy in certain high-temperature applications such as petrochemical furnaces). Cobalt-based alloys, on the other hand, may be used. Typically, the primary tradeoff is economic. When comparing a high initial cost to the life cycle cost of a conventional heat resistant alloy, the best long term value can be determined.

High temperature applications requiring heat resistant materials are common in industry. Power plants, mineral pyro processing (for example, cement, lime, and iron ores), waste incineration, petrochemical processing, steel and non-ferrous mills, metal processing (including heat treating), and glass making / forming are among the applications.

The strength at high temperatures is frequently the first consideration when selecting an alloy for a high-temperature application. However, because many high temperature applications occur in harsh corrosive environments such as chemical plants, strength is not the only important consideration. (As a result, the corrosion module in this newsletter series may be useful when evaluating elevated temperature material options.)

Conventional tensile testing at elevated temperatures demonstrates the relative strength of alloys in the short term. Additional properties such as stress rupture strength, creep strength, and/or thermal fatigue strength would be considered by the designer for an alloy's long-term performance.

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Heat-Resistant Alloy Tradeoffs

When discussing heat resistant casting applications, there are clear 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 useful to categorise the alloys into five commonly used groups. The following introduction provides some context and a general framework for classifying alloys under consideration for an application.

Rupture of Stress

The first consideration in alloy selection is typically stress rupture strength. Rupture strength is the minimum stress that will cause failure during the expected design life of the equipment. In petrochemical applications, for example, this is 100,000 hours (11.4 years). These values are typically extrapolated from shorter-duration tests.

Thermal Exhaustion

Thermal fatigue needs to be taken into account when designing components that will be subjected to thermal cycling or thermal shock while in use. The condition known as fatigue occurs when alternating loads cause failure to occur more quickly and at lower stresses than would be anticipated under a steady load. The phenomenon known as thermal fatigue occurs when the stresses are predominantly brought on by restricted expansion or contraction. These could be brought on by heat gradients inside the component or external restrictions. One of the areas in which technical metallurgical advice will benefit users is in the selection of alloys, which is still mostly reliant on experience.

Welding

Not all applications will call for the component to be welded, but when one does, additional care must be taken to ensure that the welded assembly meets the requirements. For instance, certain nickel alloys and cobalt alloys are highly challenging to weld, therefore it is crucial to balance the features those materials can offer with the difficulty of producing the finished product. check for more details Fogging testing lab

Characteristics Of Heat-Resistant Alloys That Are Desirable

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