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Calculation of pitting resistance equivalent numbers (PREN)

Introduction

Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels, based on their chemical compositions.
The PREN (or PRE) numbers are useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.
Actual or specified range compositions can be used and usually involve chromium, molybdenum and nitrogen in the calculations.

Affect of alloying elements on pitting resistance

These are 'linear' formulas, where the molybdenum and nitrogen levels are 'weighted' to take account of their strong influence on pitting corrosion resistance.

They typically take the form

PREN = Cr + m Mo + n N

where 'm' and 'n' are the factors for molybdenum and nitrogen.

The most commonly used version of the formula is

PREN = Cr + 3.3Mo + 16N

Some formulas weight nitrogen more, with factors of 27 or 30, but as the actual nitrogen levels are quite modest in most stainless steels, this does not have a dramatic effect on ranking. Tungsten is also included in the molybdenum-rating factor to acknowledge its affect on pitting resistance in the tungsten bearing super-duplex types (1.4501). A modified formula is then used:

PREN = Cr + 3.3(Mo +0.5W) + 16N

Calculated pitting resistance numbers

Nitrogen ranges are not specified in standards such as BS EN 10088-1 for all but specific grades, such as 1.4311 (304LN), 1.4406 (316LN) austenitics. In contrast all the duplex grades have specified nitrogen ranges. It can then be misleading to use just specified ranges as the residual nitrogen in commercially produced austenitics will benefit the pitting resistance.

The table below shows a range of calculated PREN values for comparison. A full theoretical range is shown, using a combination of the lowest and highest specified values for a selection of ferritic, austenitic and duplex grades.
The values are rounded for convenience of display.
The PREN values for commercially available grades will of course lie somewhere between these minimum and maximum values and so commercially available steels in grades 1.4410 and 1.4501 are likely to have actual PREN values over 40.

Grades with a PREN of 40 or more are known as 'super' austenitics or 'super' duplex types, depending to which basic family they belong.

PREN = Cr + 3.3Mo + 16N

Note The nearest AISI grade is shown, where appropriate, otherwise a typical trade name used by some of manufacturers is shown.

NS - Not specified

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