Mechanical strength characteristics of nickel base alloys

The potential to withstand the combined onslaught heat and corrosion makes nickel base alloys a supreme choice for aggressive elevated temperature conditions. Nickel alloys have extreme application for chemical plant systems subjected to corrosive process streams at high temperatures up to 1000oF. In various cases, high strength, chemical resistant nickel alloys are recommended, if not only, practical material for hostile conditions beyond the capability of austenitic and superaustenitic stainless steels. However costlier than iron alloys, supreme performance properties of heat and corrosion resistant nickel base alloys usually makes them a very economical choice with long term service.

Heat and corrosion resistant alloys have wide applications in chemical processing. Every alloy has specific UNS number. Inconel alloys 600, 601 and 625 are commonly used. The physical properties of nickel alloys are similar to series 300 Chromium-Nickel stainless steels. For each alloy heat conductivity and expansion properties significantly vary and should be considered in equipment design. The mechanical characteristics ofnickel alloy Monel 400 bars are extreme in strength and ductile characteristics.

At 1500oF, nickel alloys retain 45 – 75% of their room temperature yield strength whilst stainless steels only retain 20 – 35% of strength. Stainless steel lose their valuable strength at temperatures about 2000oF and higher. Nickel alloys can still perform significantly for moderately stressed parts. For instance 1000 hour rupture strength at 2000oF is about 1 ksi for Inconel alloys 600, 601 and 1.4 ksi for Inconel alloy 617.

ASME boiler and pressure vessel code contains permitted stress for the alloys inspected except for Haynes 214.

Another essential characteristic in alloy choice for high temperature applications is metallurgical stability that is also called as heat stability. It refers to resistance to develop brittle microstructural phases or precipitates upon aging that after extensive exposure at high temperatures. It is called age embrittlement manifests  basically a decreased ductility and toughness and can also impair corrosion resistance.

Some alloys such as Inconel alloy 600 and Inconel 601 are virtually immune to age embrittlement, many undergo different levels of impairment. Among those adversely influenced is Inconel alloy 625 that may experience a remarkable drop in ductility and impact strength when subjected in the range of about 1200oF – 1400oF. At higher temperatures, these characteristics are partly restored due to dissolution of brittle precipitates. System failures featured to decreased ductility and hardness are infrequent that can be ascribed to the very high beginning properties usually of unaged nickel alloys.

The most prevalent form of attack in high temperature chemical processing environment is gaseous corrosion, usually oxidation, sulfidation and halogenations. Various forms o damage occurred in extreme high temperature conditions are carburization, nitridation and hydrogen corrosion. Those are not classed as corrosion in the traditional sense of word, as there is no metal loss or surface recession. Instead, damage manifests as metallurgical or mechanical impairment – usual in the form of embrittlement.

The directional influence of alloying elements on retarding or exacerbating high temperature chemical corrosion of nickel base alloys is evaluated. The influence of chromium, molybdenum, copper, tungsten, silicon and aluminum can be either suitable or harmful, depending on the specific exposure conditions, significantly temperature and reducing versus oxidizing condition.