Should I choose Annealed, Hot Rolled or Strain Hardened Condition of Nitronic 50?

22nd May 2020

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Nitronic 50 is a higher-performance austenitic stainless steel that provides approximately twice the yield strength of regular 3xx series stainless steels and improved corrosion performance. It is widely used in a variety of marine and industrial applications. This alloy is available in several conditions, Solution Annealed, Hot Rolled or Strain Hardened.

The first option is the Solution Annealed condition, which is the easiest condition to machine. It is supplied with a minimum yield strength of 55ksi (380N/mm2). It retains its excellent impact toughness and corrosion resistance, but its mechanical strength will limit applications. However, its ease of machining makes it suitable to produce more intricate components, including parts for connectors, pump and valve trim. The sales of Nitronic 50 Annealed are far lower than Nitronic 50 High Strength as the machinability comes with the compromise of either relative cost or performance (strength, corrosion resistance) versus alternative alloys.

The second option is Nitronic 50 High Strength, which is offered in either the Hot Rolled or Strain Hardened condition dependent upon the diameter. The larger diameters can attain the higher strength whilst still falling below the maximum hardness level (HRC<35) specified in NACE MR-1075 (ISO 15156) through hot rolling – using lower than normal rolling temperatures to influence the microstructure. Hot rolling at high temperatures results in dynamic recrystallisation, whereby the microstructure is broken down by the rolling action but can recrystallise due to the favourable thermodynamic conditions of high temperature. This results in a more refined and uniform microstructure, with good toughness and consistency through the bar. Hot rolling at lower temperatures reduces the extent of recrystallisation, effectively imparting a degree of cold working which increases the strength but reduces the elongation slightly. Therefore, a hot rolled product achieves a minimum yield strength of 60ksi 415N/mm2) above 3” diameter, 75ksi (515N/mm2) below 3” diameter, and 105ksi (725N/mm2) below 2” diameter. Hot rolling at lower temperatures, so-called warm-working, achieves 105ksi (725N/mm2) across a wider dimensional range.

 

Strain hardening is a form of cold-working, and so greatly increases the strength of the alloy but at the expense of elongation. A minimum yield strength of 125ksi (862N/mm2) is achieved. Strain hardening is undertaken by drawing the hot-rolled bars through a die to a smaller diameter, undertaken at room temperature. Generally speaking, the greater the amount of cold working, the greater the increase in yield strength. Drawing is practically restricted to bars of less than 2 ½” in diameter, as at this size the strength of the bar becomes too high to draw it any further. For smaller diameters, we have found that the strain hardened condition achieves more consistent properties and lower through-bar hardness.

Due to its excellent impact toughness and good corrosion resistance, one of its most common applications for Nitronic 50 High Strength is for the manufacture of valve stems.

Langley Alloys carries a comprehensive stock of Fermonic 50 (XM-19, UNS S20910, Nitronic 50, 1.3964) in sizes from 3/4″ (18.9mm) to 10″ (254mm) diameter.

Nitronic is a trademark of AK Steel Corporation. 

Fermonic is a trademark of Langley Alloys Ltd.

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