Evaluation of Heavy Section, High Strength Steel Castings for Fitness for Service

The project highlighted here was the result of qualification tests on weld repairs for a heavy section, A 487 grade 6B (95 Ksi min yield) casting, which produced very low Charpy (CVN) impact values. The customer contacted Metallurgical Consulting to perform a thorough evaluation of the high strength steel casting set because of the critical nature of the application.

The metallurgical services provided included an evaluation, which included tensile tests, nil ductility tests, CVN test, hydrogen notch sensitivity tests, as well as an evaluation of the microstructure. CVN tests produced 4 ft-lbs at 0°F and 26 ft-lbs at 300°F, while tensile tests at room temperature were brittle and produced 166 ksi yield strength, with tempering at 1200°F having little effect on the toughness. The high aluminum content and slack quenching without tempering produced a bainitic microstructure with brittle grain boundary films. These problems were determined to be the reason for the poor toughness which could not be recovered by reheating. This combination of factors resulted in the recommendation that the castings be scrapped.

Conclusions on the quality of this set of castings raised concerns about the quality and reliability of earlier cast sets that are currently in service. The resulting review involved examining prior casting records, exploring previous and anticipated service, and working with the client to develop a finite element analysis of the rails in operation. It also resulted in the development of a fracture control plan using Charpy impact test data correlated with fracture mechanics properties.

This review and fracture mechanics analysis produced the conclusion that rails already in service had been tempered and were safe for continued operation. The conclusion was supported by the fact that the first set of rail castings had the worst coupon properties (they were tempered) but had several years of successful service under the harshest service conditions to be expected.

To learn more about this project, or the processes used to complete it, contact Metallurgical Consulting directly.

Images From The Metallurgical Services Provided For This High Strength Steel Casting

(click on thumbnail to enlarge)

$Figure 1: Some tempering occurred in the 300°F Charpy and the fracture was mostly microvoids but also low energy.$ Figure 1: Some tempering occurred in the 300°F Charpy and the fracture was mostly microvoids but also low energy.

Figure 1: Some tempering occurred in the 300°F Charpy and the fracture was mostly microvoids but also low energy.

$Figure 2: Tensile fracture at room temperature was brittle.$ Figure 2: Tensile fracture at room temperature was brittle.

Figure 2: Tensile fracture at room temperature was brittle.

$Figure 3: Charpy fracture at 300°F was energy.$ Figure 3: Charpy fracture at 300°F was energy.

Figure 3: Charpy fracture at 300°F was low energy.

$Figure 4: The tensile fracture had multiple modes of fracture.$ Figure 4: The tensile fracture had multiple modes of fracture.

Figure 4: The tensile fracture had multiple modes of fracture.

Figure 5: Grain boundaries carbides can be seen in this polished and etched microstructure.