Fig.8 Two spiral power springs from a textile machine. Spring at left is an acceptable part, whereas spring at right took an excessive set(the inner end of the spiral is 30 out of position) because of insufficient yield strength and a decarburized surface layer. The material in the satisfactory spring had a hardness of 45 HRC, while the material in the spring that failed had a hardness of 41.5 HRC. This represents a 10% disparity in tensile and yield strengths between the two springs. The spring that failed had a 0.08 mm(0.003 in )thick surface layer of partial decarburization that further weakened the region of the cross section where maximum stresses are developed in the spring under load. The decarburized layer, which had a lower yield strength than the bulk material, yielded excessively during presetting; therefore the spring did not attain its specified shape in the free state following this operation Another material deficiency that can lead to deformation failure is variability in response to heat treatment among parts in a given production lot. Certain alloys, particularly hardenable low-alloy steels and some precipitation-hardening alloys, can vary in their response to a specified heat treatment because of slight compositional variations from lot to lot or within a given lot. This can result in some parts having too low a strength for the application even though they were properly eat treated according to specification Remedies for variability in response to heat treatment usually involve changes in the heat treating process, ranging in complexity from tailoring the heat treating conditions for each lot or sublot to making a small adjustment in the heat treatment specification. Material composition and microstructural specifications can also be tightened to provide more uniform heat treatment response. Experiments on each lot or sublot are almost al ways needed to establish parameters when heat treating conditions are tailored An adjustment in heat treating conditions was successful in avoiding variation in properties among sublots of heat treated AISI type 631(17-7 PH) stainless steel Belleville washers. Two of these washers--one of which was from an acceptable sublot and the other from a deficient sublot-were subjected to examination. The washer from the acceptable sublot had developed the required hardness upon solution heat treating at 955C(1750F), followed by refrigeration at-75C (-100 F)and aging. The other washer was soft after an identical heat treatment and yielded under load (flattened). The microstructure of the acceptable washer was a mixture of austenite and martensite, the structure of the washer that flattened consisted almost entirely of austenite Previous experience with 17-7 PH stainless steel indicated that some alloy segregation was not unusual and that relatively minor variations in composition could affect response to heat treatment, perhaps by depressing the range of martensite transformation temperatures to a variable degree. As noted in Ref 5, the solution-treating temperature has a marked effect on the martensite start(Ms)temperature in the precipitation-hardening stainless steels that are austenitic as solution annealed and martensitic as aged(17-7 PH, AM-350, AM-355 and PH15-7Mo) Consequently, although it never was clearly established whether temperature variations inside the solution-treating furnace or minor variations in composition were responsible for the observed variability in properties of the 17-7 PH Belleville washers, all sublots attained the required strength when the solution-treating temperature was lowered to 870C(1600F) Faulty Heat Treatment. Mistakes made in heat treating hardenable alloys are among the most common causes of premature failure. Temperatures that are either too high or too low can result in the development of inadequate or undesirable mechanical properties. Quenching a steel part too fast can crack it; quenching too slowly can fail to produce the required strength or toughness. If parts are shielded from a heating or cooling medium, they can respond poorly to heat treatment. as discussed in this section Two hold-down clamps, both from the same lot, are shown in Fig 9. Both clamps were bowed to the same degree after fabrication, as intended, but the clamp at the bottom flattened when it was installed. A small percentage of the clamps, all of which were made from hardened-and-tempered 1070 steel, deformed when a bolt was inserted through the hole and tightened. The clamp at top in Fig. 9 was acceptable, with a microstructure of tempered martensite and a hardness of 46 HRC, the clamp at bottom, which deformed, had a mixed structure of ferrite, coarse pearlite, and tempered martensite and a hardness of only 28 HRO Fig.9 Two hardened-and-tempered 1070 steel hold-down clamps. The clamp at top was acceptable. The clamp at bottom was slack quenched because of faulty loading practice Thefileisdownloadedfromwww.bzfxw.comFig. 8 Two spiral power springs from a textile machine. Spring at left is an acceptable part, whereas spring at right took an excessive set (the inner end of the spiral is 30° out of position) because of insufficient yield strength and a decarburized surface layer. The material in the satisfactory spring had a hardness of 45 HRC, while the material in the spring that failed had a hardness of 41.5 HRC. This represents a 10% disparity in tensile and yield strengths between the two springs. The spring that failed had a 0.08 mm (0.003 in.) thick surface layer of partial decarburization that further weakened the region of the cross section where maximum stresses are developed in the spring under load. The decarburized layer, which had a lower yield strength than the bulk material, yielded excessively during presetting; therefore the spring did not attain its specified shape in the free state following this operation. Another material deficiency that can lead to deformation failure is variability in response to heat treatment among parts in a given production lot. Certain alloys, particularly hardenable low-alloy steels and some precipitation-hardening alloys, can vary in their response to a specified heat treatment because of slight compositional variations from lot to lot or within a given lot. This can result in some parts having too low a strength for the application even though they were properly heat treated according to specification. Remedies for variability in response to heat treatment usually involve changes in the heat treating process, ranging in complexity from tailoring the heat treating conditions for each lot or sublot to making a small adjustment in the heat treatment specification. Material composition and microstructural specifications can also be tightened to provide more uniform heat treatment response. Experiments on each lot or sublot are almost always needed to establish parameters when heat treating conditions are tailored. An adjustment in heat treating conditions was successful in avoiding variation in properties among sublots of heat treated AISI type 631 (17-7 PH) stainless steel Belleville washers. Two of these washers—one of which was from an acceptable sublot and the other from a deficient sublot—were subjected to examination. The washer from the acceptable sublot had developed the required hardness upon solution heat treating at 955 °C (1750 °F), followed by refrigeration at -75 °C (-100 °F) and aging. The other washer was soft after an identical heat treatment and yielded under load (flattened). The microstructure of the acceptable washer was a mixture of austenite and martensite; the structure of the washer that flattened consisted almost entirely of austenite. Previous experience with 17-7 PH stainless steel indicated that some alloy segregation was not unusual and that relatively minor variations in composition could affect response to heat treatment, perhaps by depressing the range of martensite￾transformation temperatures to a variable degree. As noted in Ref 5, the solution-treating temperature has a marked effect on the martensite start (Ms) temperature in the precipitation-hardening stainless steels that are austenitic as solution annealed and martensitic as aged (17-7 PH, AM-350, AM-355 and PH15-7Mo). Consequently, although it never was clearly established whether temperature variations inside the solution-treating furnace or minor variations in composition were responsible for the observed variability in properties of the 17-7 PH Belleville washers, all sublots attained the required strength when the solution-treating temperature was lowered to 870 °C (1600 °F). Faulty Heat Treatment. Mistakes made in heat treating hardenable alloys are among the most common causes of premature failure. Temperatures that are either too high or too low can result in the development of inadequate or undesirable mechanical properties. Quenching a steel part too fast can crack it; quenching too slowly can fail to produce the required strength or toughness. If parts are shielded from a heating or cooling medium, they can respond poorly to heat treatment, as discussed in this section. Two hold-down clamps, both from the same lot, are shown in Fig. 9. Both clamps were bowed to the same degree after fabrication, as intended, but the clamp at the bottom flattened when it was installed. A small percentage of the clamps, all of which were made from hardened-and-tempered 1070 steel, deformed when a bolt was inserted through the hole and tightened. The clamp at top in Fig. 9 was acceptable, with a microstructure of tempered martensite and a hardness of 46 HRC; the clamp at bottom, which deformed, had a mixed structure of ferrite, coarse pearlite, and tempered martensite and a hardness of only 28 HRC. Fig. 9 Two hardened-and-tempered 1070 steel hold-down clamps. The clamp at top was acceptable. The clamp at bottom was slack quenched because of faulty loading practice The file is downloaded from www.bzfxw.com