Improvement of Reliability of Conpressors for Domestic Refrigerators Using HFC134a

The conclusions of an experimental program involving altered tensile strength and microstructure of stainless compressor valve steel are described. The paper presents the results of measurements of ductility, corrosion resistance, bending fatigue strength and damping. Encouraging indications have been achieved concerning the possibilities to further improve the characteristics of current material. INTRODU£TION One of the essential properties of a compressor valve steel is its ability to resist the stresses, created by the repeated bending of the valve without fracturing. The material parameters which control this fatigue strength are in principle well known. For e~ample, a high static strength and compressive surface stresses contribute to improved fatigue strength, while rough surfaces and corrosion have the opposite influence. Since long, there have been two types of materials available for the manufacture of compressor valves, one being carbon steel with a composition corresponding to AISI 10g5_ The other is a martensitic chromium steel based on AISI 420 but with an addition of 1 % Molybdenum. The fatigue strength of compressor valves manufactured from these steel types has probably increas~d by the development of metallurgical processes giving ~leaner steels and valve manufacturing methods resulting in improved edge and surface properties and controlled surface stresses. On the other hand, only marginal changes have been made concerning mechanical properties of compressor valve steels since specifications were originally set. Corrosion has been a limited problem in most compressor applications why little attention has been paid to corrosion resistance of the stainless grade. As service conditions in the compressors currently are subject to change, partly as a result of new refrigerants and lubricants being introduced, a gradual substitution of carbon steel valves seems to take place, f&vouring the stainless material. In addition, compressor designers ~ave the ambition to increase the efficiency of compressors which makes for higher mechanical load on the valves. For the reasons indicated above it has been considered to be of interest to evaluate the potential for improved characteristics of stainless compressor valve steel. In this paper, the results of an experimental program will be discussed. 2 EXPERIMENTAL The tested material was Sandvik 7C27Mo2. Nominal composition and specified tensile strength of this material is given in table 1. Trial strips with thicknesses ranging between 0.152 mm (.006") and 0.508 mm (.020") were hardened and tempered to tensile strength levels above the upper limit of the currently valid specification range. The heat treatment was made in the normal production equipment. These experimental strips were characterized concerning their mechanical properties. As elevated tensile strength normally causes reduction of the ductility of a steel, extra emphasis was put on ductility measures. Some of the strips were exposed to fatigue testing. The resistance to pitting corrosion was evaluated and an attempt was made to measure the damping properties of the material. Microstructures were assessed.