API SC 6 Casting Research Project Evaluation of PSL Quality and Manufacturing Activities for Casting for Service Integrity

Review Available Literature Concentrated on suitability for API 6A PSL 3 service

Introduction A review of the available literature reveals that both castings and forgings are capable of meeting API 6A service requirements. It is often stated that a forging is a better product because it is formed or worked during the manufacturing process. It also assumed that castings are inferior because they may contain deleterious porosity. Neither statement is totally true.

Definition ForgingCasting A process by which metal is heated and shaped by plastic deformation by suitably applying compressive force. Forgings are anisotropic. Mechanical properties are higher in the longitudinal direction, lower in the transverse direction. A process of pouring liquid metal into a mold. Castings are isotropic. Mechanical properties are the same in all directions.

Casting vs. Forging Castings are best suited to complex shapes, custom or tailored chemistries and to applications that are subject to multi-axial stresses. In general, forgings are best suited to simple configurations that can be worked in a die or other tooling. Forgings are also suited to applications in which the principal applied stresses are the same as the direction of mechanical working.

Casting vs. Forging Both forgings and castings are vulnerable to particular types of defects. CastingsForgings Surface Discontinuities Sand Inclusions Porosity Hot Tears Shrinkage Inclusions Bursts Poor Grain Structure Laps (Folds) Seams Cold Shuts Cracks, Tears Inclusions

Mechanical Property Comparisons The test results on the next slide are from a 5 inch thick low alloy steel test casting and a five inch thick plate of rolled 4130 steel. The test results demonstrate that the mechanical properties of the cast sample are essentially the same regardless of test bar orientation. The test results of the wrought sample are lower in both the transverse and through thickness orientations, especially the ductility, which shows a significant degradation when compared to the longitudinal direction. The same directionality effects are demonstrated when comparing fatigue strength of cast and wrought alloys. However the notched fatigue properties of cast steel are actually superior to wrought steel regardless of orientation. This demonstrates that cast steel is less notch sensitive than wrought steel.

Mechanical Properties Sample Test Results Tensile (ksi)Yield (ksi)% Elongation% Red in Area WroughtCastWroughtCastWroughtCastWroughtCast Longitudinal %12.0%46.5%26.0% Transverse %11.0%33.5%21.7% Thu Thickness %11.0%13.0%24.8% Example of Grain Flow

Summary Technical data was not found that supports limiting the service conditions of PSL-3 castings. The use of improved foundry techniques that result in a much better casting are in routine use today in foundries, but would be difficult to make a requirement in a specification. o Proper casting design to optimize castability. o Utilize progressive solidification as part of the casting design to minimize porosity caused by shrinkage. o The use of solidification modeling software to validate casting design.

Next Step in the Project If desired, develop testing program designed to determine casting suitability in a high pressure, sour or corrosive environment. Consider including, as a normative reference in API Spec 6A, API Specification 20A, Carbon Steel, Alloy Steel, Stainless Steel, and Nickel Base Alloy Castings for Use in the Petroleum Natural Gas Industry o Defines requirements for casting levels, CSL-1 – CSL-4 o Requires qualification of a casting class and rules for production based on qualification. o Structured similar to Spec 6A; a higher CSL has an increased level of technical, quality and qualification requirements. o The highest specification level, CSL-4 does not permit weld repair.