API RP 6HT-2013 pdf download.Heat Treatment and Testing of Carbon and Low Alloy Steel Large Cross Section and Critical Section Components.
1 Scope This recommended practice (RP) may supplement the API equipment specifications for carbon and low alloy steel large cross section and critical components. The recommend practice described herein suggests the requirements for batch-type bath quench and water spray quench-type heat treating practices. 2 Normative References Standards referenced in this specification may be replaced by other international or national standards that can be shown to meet or exceed the requirements of the referenced standard. Manufacturers who use other standards in lieu of standards referenced herein are responsible for documenting the equivalency of the standards. Referenced standards used by the manufacturer may be either the applicable revision shown in Section 2 and herein or the latest revision. When the latest edition is specified it may be used on issue and shall become mandatory six months from the date of the revision. API Specification 6A, Specification for Wellhead and Christmas Tree Equipment ASTM A255 1 , Standard Test Methods for Determining Hardenability of Steel NACE MR0175 2 /ISO 1 51 56 3 , Petroleum and natural gas industries—Materials for use in H 2 S-containing environments in oil and gas production SAE AMS-H-6875 4 , Heat Treatment of Steel Raw Materials 3 Terms and Definitions For the purposes of this document, the following definitions apply. 3.1 critical section components Any part having a cross section thickness with an equivalent round (ER) that exceeds the depth of hardenability of the alloy selected for the part. 3.2 large cross section Any part having a cross section thickness with an equivalent round (ER) greater than 5 in. (125 mm). 3.3 prolongation An extension of a piece of raw material or an extension of a production part made integrally during forging, hot working, cold working or casting for the purpose of performing mechanical testing and metallurgical evaluation. 3.4 QTC Qualification test coupon.
4 Purpose Heat treatment is a critical process that must be appropriate and controlled in order to produce parts that comply with design requirements. Per API 6A, “The properties exhibited by the QTCs shall represent the properties of the thermal response of the material comprising the production parts it qualifies. Depending upon the hardenability of a given material, the QTC results might not always correspond to the properties of the actual components at all locations throughout their cross section.” The specified mechanical properties may not necessarily be required or achieved through the entire section thickness of the production part(s). These procedures are intended to provide the manufacturer and end user with a means of ensuring that the qualification test coupon (QTC) is more representative of the mechanical properties in a large cross section component than can be expected with a standard API equipment specification QTC. Furthermore, these procedures are intended to provide to optimize the heat treatment and heat treatment response of large cross section components, thereby insuring that the component has the required mechanical properties at the depth below the surface established by the manufacture at all critical locations. It should be noted that the required mechanical properties as established by the manufacturer may be different from the mechanical properties required by the API equipment specification. This recommended practice is intended to supplement the heat treatment and testing requirements found in the API equipment specification and not to replace them altogether. 5 Application This recommended practice is intended for use on large cross section components being manufactured for conformance to API equipment specifications.
6 Recommended Heat Treating Practices 6.1 General Heat treating may be defined as the controlled heating and cooling of a metal in order to obtain a desired microstructure and consequently desired properties. Carbon and low alloy steels are the most widely used alloys in oil and gas exploration and production. One of the reasons for this is their versatility: a wide range of properties can be obtained through an appropriate heat treatment. The basis for heat treating carbon and low alloy steels is that they have several different stable crystal structures depending on the heat treatment process used. By transforming the crystal structures during heat treatment, the desired microstructure and mechanical properties can be obtained in the end product, provided the size of the product does not exceed the hardenability limits of the alloy. The most common type of heat treatment imposed on carbon and low alloy steels is a three- to four-step process consisting of austenitizing, quenching, and tempering (Q&T) or normalizing, austenitizing, quenching, and tempering (N-Q&T). The austenitizing cycle consists of heating the steel up to a temperature high enough to completely transform its microstructure into austenite (typically about 1 500 °F to 1 700 °F or 81 6 °C to 927 °C for most common low alloy steels). Austenite is a phase of steel having a face-centered cubic structure.