What Is PREN? How to Calculate Pitting Resistance

Chapters

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1. 010:00PREN formula: chromium, molybdenum, tungsten and nitrogen

   What is PREN, and how do you calculate it? One number from three elements decides many stainless shortlists. Here is the math, and what the number cannot prove. PREN means pitting resistance equivalent number. Take chromium, in mass percent. Add 3.3 times molybdenum plus half the tungsten. Add 16 times nitrogen. It screens chemistries; it is not a corrosion test.

2. 020:25How to calculate PREN step by step: worked example

   Take 25 percent chromium, 4 molybdenum and 0.27 nitrogen: illustrative inputs. Chromium counts as 25.00. 3.3 times 4 molybdenum is 13.20; 16 times 0.27 nitrogen is 4.32. Add them: PREN is 42.52. Drop nitrogen from 0.27 to 0.24, nothing else. PREN falls to 42.04. The 0.48 gap is 16 times 0.03: state which nitrogen you used.

3. 031:03Why a grade's chemistry range gives a PREN range: 2507, 254SMO, 316L vs 317L

   A grade name gives a chemistry range, so it gives a PREN range. Across Junda's published 2507 range, S32750, PREN runs from 37.74 to 47.62. 254SMO, S31254, runs from 42.18 to 45.47. So the grade alone is not a PREN value: ask which data the number came from. Molybdenum alone can mislead too. 316L runs from 2.00 to 3.00 percent molybdenum; 317L from 3.00 to 4.00. The ranges touch at 3.00, so one element does not make two grades equal. Check the full chemistry against the ordered standard. Every PREN has a data basis. A grade reference table gives typical chemistry. A specification limit gives the allowed range. A heat certificate gives the measured analysis; if nitrogen is missing, don't fill in a nominal value.

4. 042:12What PREN does not prove: heat treatment, phases, surface, service

   PREN reads the chemistry, not the finished pipe. The grade marking tells you what was ordered. The standard sets chemistry, heat treatment and tests. Surface, geometry and fabrication are not in the formula at all. Think in three stages of evidence. Chemistry screening, like PREN, narrows the candidates. Specified tests evaluate the material under stated conditions. Engineering assessment decides suitability for the actual service. For duplex, ASTM A923 notes that chemical compliance does not prove the absence of detrimental phases.

5. 052:52How to write a PREN requirement into a pipe enquiry

   If your project sets a PREN requirement, write it out in full. Name the formula and its coefficients. Name the data basis: reference, specification or heat certificate. Give the acceptance threshold, approved by the responsible engineer. And list the documents you need with the delivery. Junda reviews PREN and chloride service conditions for each enquiry: send your grade, standard and PREN requirement.