THE EFFECTS OF PHOSPHORUS ON THE MECHANICAL PROPERTIES OF IRON

Abstract

Fe-P alloys with nominal compositions of 0.05, 0.1, 0.2, and 0.6 wt pct P were deformed in compression at temperatures between 77 and 295 K at strain rates between 2.5 • 10-4 and 3.5 • 10-2 per see. The effect of grain size on the yield strength was investigated over a range of about ASTM 7 to 3.5. The results show that the yield strength of iron is increased 40 to 50 pet in the 0.05 and 0.1 P alloys at 295 K without affecting the notch impact transition temperature. At temperatures of 200 K and below solid solution softening occurs in all the alloys except for 0.6 pet P. The degree of softening at a given temperature is a function of the strain rate and grain size, being more pronounced at higher strain rates and larger grain sizes. THIS investigation is concerned with the effect of phosphorus on the low-temperature deformation of iron and is part of a larger program dealing with the influence of alloying elements on the mechanical properties of iron. Previous investigations have been primarily concerned with the effect of phosphorus on the toughness of iron and have shown that phosphorus, like oxygen and nitrogen, produces intergranular weakness in iron to an extent depending on the concentration of carbon and nitrogen in the alloy. '-4 Additional studies have concentrated on analysis of precipitation in Fe-P alloysf '6 On an atom basis, the strengthening produced by phosphorus in iron is greater than that obtained from any other substitutional solute, v Therefore, it appeared desirable to investigate more thoroughly the mechanical properties of Fe-P alloys, and, in particular , the effects of temperature, strain rate, grain size, and phosphorus concentration on the low-temperature mechanical properties. MATERIALS AND PROCEDURES Fourteen kg ingots of nominal compositions 0.05, 0.1, 0.2, and 0.6 wt pct P were made by adding ferro-phosphorus to Plastiron (Type 104A). The ferrophos-phorus was prepurified by melting under an FeO slag. The Plastiron was melted and vacuum-carbon-deox-idized prior to adding the purified ferrophosphorus. The compositions of the alloys are given in Table I. The ingots were hot-rolled between 1366 and 1172 K to 12.7 mm thick plate and air cooled. Compression specimens 12.7 mm high and 6.35 mm in diam were made by cutting 12.7 mm sq sections from the hot-rolled plate and cold swaging to 7.15 mm diam rods. These compression specimens had their cylinder axes parallel to the rolling direction. They were given the recrystallization treatments shown in Table II after final machining. Half-width Charpy V-notch specimens 5 by 10 mm and 55 mm long with a 45 deg notch 2 mm deep were machined from plate which had been cold-rolled from 12.7 to 6.35 mm in thickness. The long dimension of these specimens was parallel to the roll-W. A. SPITZIG is Senior Scientist, Edgar C. ing direction and the axis of the notch was perpendicular to the plate surface. Table HI shows the recrys-tallization treatments and grain sizes of the Charpy specimens. All specimens were heat-treated in evacuated vycor capsules then quenched into brine. They were tested within 24 hr of heat treating; however, holding at 295 K for longer periods (several weeks) had no apparent effect on mechanical properties. All of the alloys and the iron had equiaxed ferrite structures; electron micro-scopy did not reveal any precipitates at grain boundaries or within grains. Grain sizes were determined by counting the number of grains in a given area of representative micrographs. Compression tests were conducted on a Baldwin-Southwark machine at strain rates of 2.5 • 10-4, 3.5 • 10-3, and 3.5 x 10-2 per sec at temperatures of 295, 250,200, 148, and 77 K, as described previously. 8 Strain rates were determined from slopes of deflection-time curves. Two or more identical specimens were tested at each strain rate and temperature and the average value of each mechanical property is reported. When lower yield points were not observed, a 0.2 pct offset stress was taken as the yield stress. At 77 K, where twinning was pronounced in iron and the alloys, the uniform strain-hardening portion of the stress-strain curve was extrapolated to the elastic line to obtain a value for the yield stress. EXPERIMENTAL RESULTS Notch Impact Tests The notch impact properties of iron and the alloys are shown in Fig. 1. The transition temperature for iron and the 0.05 and 0.1 P alloys is about 230 K. Small additions of phosphorus have a negligible effect on transition temperature in these rapidly-cooled specimens. The 0.2 and 0.6 P alloys had transition temperatures of about 285 and 465 K, respectively. These results are in qualitative agreement with those reported previously for Fe-P alloys with low interstitial contents , 4 although a quantitative comparison of results is not possible because of differences in heat treatments , microstructures and grain sizes. When the impact specimens broke in a brittle manner iron and METALLURGICAL TRANSACTIONS VOLUME 3, MAY 1972-1183