CA6NM: New developments based on 20 years' experience

Development of CA6NM cast steel is reported and the present state of the international standards is discussed. The influence of the argon oxygen decarburization (AOD) process on CA6NM is shown by means of a statistical evaluation of preliminary quality-control test specimens from the four year period 1975-1979. The influences of reducing the carbon content to 0.030 percent and the sulfur content to 0.004 percent on the impact energy are given special attention. The impact energy values were improved by 18 to 73 percent, depending on test specimen shape and test temperature. This resulted in a high average value (Charpy V-notch, +20°C: 152 ′ 21 J) even on heavy-walled castings. A further statistical evaluation verifies that CA6NM cast steel with a strength level of 950 MPa also has excellent impact values (Charpy V-notch, +20°C: 67 ′ 19 J). The increase in steel quality due to the AOD refining process becomes evident especially for large cross sections. These have an improved primary structure. By using massive stepped test blocks with step thickness of up to 500 mm, it is shown that the drop in reduction-of-area and elongation values from surface-to-core location observed in earlier investigations on basic electric-arc furnace products disappears completely. In addition, the corrosion fatigue limit values in the core of 500-mm sections are up to 30 percent higher than for good are furnace steel. Since the phosphorus level in the AOD test block was purposely aimed at a relatively high level of 0.021 percent, this effect must be attributed to the very low nitrogen, hydrogen, oxygen, and sulfur contents in the AOD steel. As a result of the higher degree of cleanliness, the Charpy V-notch upper-shelf energy level as well as the transition temperature show marked improvement even in the core location. The optimum welding conditions are described. The importance of welding electrode quality control and the question of post-weld heat treatment are discussed. To supplement the documentation of the material's potential for low-temperature applications, technical base material data are presented. The conditions for obtaining satisfactory weld properties at temperatures of -110°C, using electrodes of composition similar to the base material, are described. The procedures are applicable for fabrication and repair welding. In light of a possible replacement of austenitic-ferritic castings by CA6NM for parts used in primary cooling loops of nuclear reactors, it is shown that heats with a high initial impact energy undergo in principle the same long-term aging process which occurs for predominately austenitic cast steel.