Quality Issues: Steel Castings For Off-Shore Application

Introduction

A large order for steel castings for off-shore application was behind schedule as a result of casting quality issues. The castings were made in a low alloy steel and heat treated to provide a combination of strength and ductility. Following machining, the castings were to be case hardened for wear resistance, final machined and inspected and shipped to be assembled.

Quality Problems

There were three main quality problems associated with the castings :

• Shrinkage voids revealed after machining
• Weld and heat affected zone HAZ cracking
• Cracking after flame hardening

Expert Analysis

Shrinkage Voids

It is not uncommon for large steel castings to contain small voids at the center of the cast sections as a result of the solidification process and shrinkage characteristics of the alloy. These voids do not have any deleterious affects on casting performance. However, due to the amount of machining that was done, some of these voids were revealed at the machined surface and were repaired prior to case hardening.

When the consultant typically becomes involved with such a project, a review is made of the casting risering system with the aid of computer simulation. Following this review, changes are made such that the shrinkage voids are minimized or located to a non-critical section of the casting. However, when the consultant became involved with the project, all the castings for the contract had already been made. The only possible remedy to the shrinkage issue was to excavate the defects and weld repair. The repair procedures that had been previously used by others were unsuccessful at producing a crack-free weld.

Weld Repair Procedure

The consultant developed a successful weld repair and inspection procedure. The defects were excavated using small carbide grinders. The cavities were then visually examined to ensure that the shrinkage voids had been removed. Due to the mass of the casting and its alloy content, it was critical to slow the cooling rate during and after welding to minimize the tensile stresses and to reduce the hardness of the HAZ. This was done by uniformly preheating the casting with multiple torches on a positioner table. The casting was covered in welding blanket to insulate and reduce temperature gradients. The casting was heated to a temperature of 500F at a very slow rate (50F / hour). Repairs were conducted using the GTAW process in the flat position. The filler metal was similar to AISI 8620. The initial passes were made by butter technique and the same filler wire used for the remainder of the cavities. Proper chipping and cleaning were performed after each stringer pass and interpass temperature was measured using an infra-red gun. Following welding, the casting was slow cooled to ambient temperature using torches and welding blanket to control rate of cool. The weld was ground and 48 hours later was non-destructively tested by magnetic particle MT using the wet fluorescent method. The welds were then non-destructively tested by ultrasonic inspection including dual element transducers and shear-wave types.

Case Hardening Procedure

Previous castings had been flame hardened but circumferential cracks had formed following this procedure. The consultant recommended that changes were made to the process as follows :

• Prior to flame hardening, and after machining and welding, the castings were stress relieved in a furnace at a rate of 100F/hour to 1050F. The castings were held at that temperature and furnace cooled. They were checked by MT and dimensionally checked to ensure that distortion had not occurred.
• Flame hardening was modified to include a longer heat treat of the heavy section prior to flame hardening. This resulted in reduced thermal shock and a lesser heat sink.

All castings produced to these procedures required no major upgrade.

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