The Development of New Materials for Nonmagnetizable Retaining Rings and Other Applications in the Power Generating Industry

Abstract

Long before the first forged retaining rings were manufactured, our company, Schmiedewerke Krupp-Klöckner, was concerned with the development of nonmagnetizable steels for the manufacture of retaining rings. In addition to our company, only a few manufacturers are manufacturing rings that meet the requirements of present-day specifications. As various steel brands developed beginning early in this century, the requirements of our customers concerning the mechanical properties and the corrosive environmental conditions became greater and greater. This led us in the late seventies to a material that meets present and future requirements regarding both mechanical properties and full corrosion resistant behavior. This paper will show the main steps in the development of retaining ring production by our company. Today we use as starting material for our retaining rings cast electrodes of electric furnace steel which are remelted in our electroslag remelting plant. The ingot undergoes several forging and reheating operations to produce a ring blank. The blanks are rough turned, solution heat-treated, and then work-hardened so as to obtain the desired 0.2% proof stress. This operation is followed by a stress-relieving heat treatment and a finish-machining operation. This production requires continuous monitoring of the production procedure as well as extensive in-process and final tests. In the last 30 years, about 18 000 retaining rings have been manufactured from a material with 0.5% carbon, 18% manganese, 4.5% chromium, and 0.1% nitrogen. Physical and mechanical properties could be met with this material even in customers' advanced specifications. There is only one disadvantage: The material is susceptible to stress corrosion cracking. Some failures of retaining rings in recent years were due to the occurrence of stress corrosion cracking. The best way to avoid stress corrosion cracking is to use a nonsusceptible material. Such a material has been developed for the manufacture of retaining rings in our company. Up to now we have produced more than 400 rings with 0.2% proof stresses up to 1250 N/mm2. This paper will show the chemical analysis, mechanical properties, residual stresses, stress corrosion properties, susceptibility to hydrogen-induced cracking, fracture toughness properties, sensitization at elevated temperatures, and stress relaxation behavior. Even with this extraordinary material, we have not stopped the development of retaining ring material. Three years ago our company installed the only full-size pressure electroslag remelting equipment in the world. With this equipment we can increase the nitrogen content of any material above the normal solubility limit. This will lead to higher 0.2% proof stresses and more homogeneous mechanical properties without losing the corrosion-resistant behavior of the new retaining ring material.