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51314-4298-Failures of 17-4 pH Steel Parts in Non Sour Enviroments

This paper presents a failure of a material that was overloaded during testing although the raw material was approved based on certified mechanical properties. The failure was due to unsuitable microstructures.

 

Product Number: 51314-4298-SG
ISBN: 4298 2014 CP
Author: Annelise Zeemann
Publication Date: 2014
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17-4 PH material is a Cr-Ni low carbon martensitic stainless steel containing Cu and Nb precipitation hardened elements to be solution annealed and aged (or double aged) to acquire different levels of strength usually higher than 100 Ksi YS adopted for parts like stems springs and fasteners among others where high mechanical properties together with corrosion resistance is required.The corrosion resistance is frequently the most important property of a stainless steel but it is also very difficult to assess because this property is very dependent on the actual environment sometimes much more complex than the one adopted as the design environment.In subsea oil and gas applications non sour environments the adoption of this alloy steel depends on whether seawater stagnant conditions are presented since this material is not highly resistant to pitting or crevice conditions. But recognizing this stagnant conditions does not seem to be easy since several failures occurred in spring and stem parts which metallurgical details are presented in this paper.Another important aspect to be considered in subsea applications is the relationship between microstructures and hydrogen generated in cathodic protection (CP). Depending on the manufacturing conditions or on the manufacturer knowledge (or lack of) these materials may achieve hardness values higher than the allowed for cathodic protection conditions and this paper also presents failures in CP applications.The product form (cast or wrought) the size and the adopted heat treatment cycles are among the main differences in oil and gas applications of this material that exhibit a wide variation in chemistry metallurgical structures and properties obviously affecting the behavior of 17-4 PH parts mainly in subsea conditions. This paper presents also results of metallurgical quality control of cast and wrought heats of raw material evaluated in the last 15 years.

Key words: 17-4PH, castings, wrought products, aging, microstructure, retained austenite, delta ferrite

 

17-4 PH material is a Cr-Ni low carbon martensitic stainless steel containing Cu and Nb precipitation hardened elements to be solution annealed and aged (or double aged) to acquire different levels of strength usually higher than 100 Ksi YS adopted for parts like stems springs and fasteners among others where high mechanical properties together with corrosion resistance is required.The corrosion resistance is frequently the most important property of a stainless steel but it is also very difficult to assess because this property is very dependent on the actual environment sometimes much more complex than the one adopted as the design environment.In subsea oil and gas applications non sour environments the adoption of this alloy steel depends on whether seawater stagnant conditions are presented since this material is not highly resistant to pitting or crevice conditions. But recognizing this stagnant conditions does not seem to be easy since several failures occurred in spring and stem parts which metallurgical details are presented in this paper.Another important aspect to be considered in subsea applications is the relationship between microstructures and hydrogen generated in cathodic protection (CP). Depending on the manufacturing conditions or on the manufacturer knowledge (or lack of) these materials may achieve hardness values higher than the allowed for cathodic protection conditions and this paper also presents failures in CP applications.The product form (cast or wrought) the size and the adopted heat treatment cycles are among the main differences in oil and gas applications of this material that exhibit a wide variation in chemistry metallurgical structures and properties obviously affecting the behavior of 17-4 PH parts mainly in subsea conditions. This paper presents also results of metallurgical quality control of cast and wrought heats of raw material evaluated in the last 15 years.

Key words: 17-4PH, castings, wrought products, aging, microstructure, retained austenite, delta ferrite

 

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