Errors in specification of material or method of processing for a part can lead to distortion failures. These errors are often the result of faulty or incomplete information being available to the designer. In such instances, the designer has to make assumptions concerning the conditions of service Example 2: Distortion Failure of an Automotive Valve Spring. The engine of an automobile lost power and compression and emitted an uneven exhaust sound after several thousand miles of operation. When the engine was dismantled, it was found that the outer spring on one of the exhaust valves was too short to function properly. The short steel spring and an outer spring taken from another cylinder in the same engine(both shown in Fig. 6) were examined in the laboratory to determine why one had distorted and the other had not Fig. 6 Valve springs made from patented and drawn high-carbon steel wire. Distorted outer spring (left)exhibited about 25% set because of proeutectoid ferrite in the microstructure and high operating temperature Outer spring(right) is satisfactory. Investigation. The failed outer spring(at left, Fig. 6) had decreased in length to about the same free length as that of its companion inner spring. Most of the distortion had occurred in the first active coil (at top, Fig. 6), and a surface residue of baked-on oil present on this end of the spring indicated that a temperature of 175 to 205C (350 to 400F) had bec reached. Temperatures lower than 120C (250F)usually do not cause relaxation(or set)in high-carbon steel springs The load required to compress each outer spring to a length of 2.5 cm(1 in. ) was measured. The distorted spring needed only 30 kg(67 Ib), whereas the longer spring needed 41 kg(90 Ib). The distorted spring had suffered 25% set, which was the immediate cause of the engine malfunction The microstructure of both springs was primarily heavily cold-drawn fine pearlite, but the microstructure of the distorted spring contained small amounts of proeutectoid ferrite. Although the composition of the spring alloy was unknown, the microstructure indicated that the material was patented and cold-drawn high-carbon steel wire. The distorted spring had a hardness of 43 HRC, and the longer spring had a hardness of 46 HrC. both hardness and microstructure indicated that the material in the deformed spring had 10% lower yield strength than material in the undeformed spring. The estimates of yield strength were considered valid because of two factors: the accuracy of the hardness testing and characteristically consistent ratios of yield strength to tensile strength for the grades of steel commonly used in spring wire Conclusions. The engine malfunctioned because one of the exhaust-valve springs had taken a 25% set in service Relaxation in the spring material occurred because of the combined effect of improper microstructure(proeutectoid ferrite) plus a relatively high operating temperature. The undeformed spring exhibited little or no set because the tensile trength and corresponding yield strength of the material(estimated from hardness measurements) were about 10% higher than those of the material in the deformed spring Recommendations. a higher yield strength and a higher ratio of yield strength to tensile strength can be achieved in the springs by using quenched-and-tempered steel instead of patented and cold-drawn steel. An alternative would be to use a more expensive chromium-vanadium alloy steel instead of plain carbon steel; the chromium-vanadium steel should be quenched and tempered. Regardless of material or processing specifications, if springs are stressed close to the yield point of the material, close control of material and processing plus stringent inspection are needed to ensure satisfactory rformance Service conditions are sometimes changed, invalidating certain assumptions that were made when the part was originally designed. Such changes include an increase in operating temperature to one at which the material no longer has the required strength, an increase in the load rating of an associated component, which the user may interpret as an increase in the allowable load on the structure as a whole, and an arbitrary increase in applied load by the user on the assumption that the component has a high enough safety factor to accommodate the added loadErrors in specification of material or method of processing for a part can lead to distortion failures. These errors are often the result of faulty or incomplete information being available to the designer. In such instances, the designer has to make assumptions concerning the conditions of service. Example 2: Distortion Failure of an Automotive Valve Spring. The engine of an automobile lost power and compression and emitted an uneven exhaust sound after several thousand miles of operation. When the engine was dismantled, it was found that the outer spring on one of the exhaust valves was too short to function properly. The short steel spring and an outer spring taken from another cylinder in the same engine (both shown in Fig. 6) were examined in the laboratory to determine why one had distorted and the other had not. Fig. 6 Valve springs made from patented and drawn high-carbon steel wire. Distorted outer spring (left) exhibited about 25% set because of proeutectoid ferrite in the microstructure and high operating temperature. Outer spring (right) is satisfactory. Investigation. The failed outer spring (at left, Fig. 6) had decreased in length to about the same free length as that of its companion inner spring. Most of the distortion had occurred in the first active coil (at top, Fig. 6), and a surface residue of baked-on oil present on this end of the spring indicated that a temperature of 175 to 205 °C (350 to 400 °F) had been reached. Temperatures lower than 120 °C (250 °F) usually do not cause relaxation (or set) in high-carbon steel springs. The load required to compress each outer spring to a length of 2.5 cm (1 in.) was measured. The distorted spring needed only 30 kg (67 lb), whereas the longer spring needed 41 kg (90 lb). The distorted spring had suffered 25% set, which was the immediate cause of the engine malfunction. The microstructure of both springs was primarily heavily cold-drawn fine pearlite, but the microstructure of the distorted spring contained small amounts of proeutectoid ferrite. Although the composition of the spring alloy was unknown, the microstructure indicated that the material was patented and cold-drawn high-carbon steel wire. The distorted spring had a hardness of 43 HRC, and the longer spring had a hardness of 46 HRC. Both hardness and microstructure indicated that the material in the deformed spring had 10% lower yield strength than material in the undeformed spring. The estimates of yield strength were considered valid because of two factors: the accuracy of the hardness testing and characteristically consistent ratios of yield strength to tensile strength for the grades of steel commonly used in spring wire. Conclusions. The engine malfunctioned because one of the exhaust-valve springs had taken a 25% set in service. Relaxation in the spring material occurred because of the combined effect of improper microstructure (proeutectoid ferrite) plus a relatively high operating temperature. The undeformed spring exhibited little or no set because the tensile strength and corresponding yield strength of the material (estimated from hardness measurements) were about 10% higher than those of the material in the deformed spring. Recommendations. A higher yield strength and a higher ratio of yield strength to tensile strength can be achieved in the springs by using quenched-and-tempered steel instead of patented and cold-drawn steel. An alternative would be to use a more expensive chromium-vanadium alloy steel instead of plain carbon steel; the chromium-vanadium steel should be quenched and tempered. Regardless of material or processing specifications, if springs are stressed close to the yield point of the material, close control of material and processing plus stringent inspection are needed to ensure satisfactory performance. Service conditions are sometimes changed, invalidating certain assumptions that were made when the part was originally designed. Such changes include an increase in operating temperature to one at which the material no longer has the required strength, an increase in the load rating of an associated component, which the user may interpret as an increase in the allowable load on the structure as a whole, and an arbitrary increase in applied load by the user on the assumption that the component has a high enough safety factor to accommodate the added load