ABSTRACT

17-4 PH material is a Cr-Ni low carbon martensitic stainless steel containing Cu as the precipitation hardening element, to be solution annealed and aged (or double aged) to acquire different levels of strength, usually higher than 100 ksi (689 MPa) Yield Strength. This material is adopted for parts like stems and springs, where any corrosion damage may affect the component functionality. The main nightmare of a designer that selects a corrosion resistant alloy (CRA) is that the future operational environment sometimes may be not exactly the one analyzed when the materials selection was carried out. The consequence of the lack of confident information may be catastrophic since any corrosion for a CRA means failure. In subsea oil and gas applications there are two main corrosive environments to be considered when selecting the material: the production fluid (sweet or sour oil), not mentioned in this paper; and seawater, where the corrosion protection may be assured by either cathodic protection (CP) or by the use of a CRA, in this case a "seawater resistant" CRA. The 17-4 PH stainless steel is not seawater resistant and when adopted for submarine conditions it is important to assure it is protected by CP. This paper presents two corrosion failures associated to pitting and crevice conditions. Another important question is related to the material resistance to CP conditions. 17-4PH is a high strength martensitic steel with limits for use in hydrogen charging conditions. This paper presents a brittle failure associated to an unexpected overpressure of a hydraulic cylinder that was hydrogen charged. Although it was created by an operational mistake, the material failed in a pressure level well below the one applied in the hydrostatic test. There is another important issue, which seems not to be well understood, related to the complexity of microstructures associated with this material. Depending on the manufacturing processes and on the manufacturer knowledge (or lack of), these materials may present large variations in the microstructures for the same specification, and some applications may not tolerate these variations. 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. A sampling of suitable and unsuitable microstructures, evaluated during quality control of this material in the last 15 years, is also presented.

This content is only available via PDF.
You can access this article if you purchase or spend a download.