Effect of phosphorus on the ductility of high strength spring steels

Abstract

Phosphorus as a tramp element deteriorates the toughness and enhances the brittle fracture in high strength steels. Even very low bulk concentrations can cause severe enrichment on the prior austenite grain boundaries. This seriously restricts the applicability of highly demanded vehicle components. Three approaches were chosen to reduce the detrimental effect of phosphorus in quenched and tempered high strength CrV‐steels for high power leaf springs: Change in the chemistry (addition of boron for repelling phosphorus from the grain boundaries), application of thermomechanical treatment (TMT) instead of conventional heat treatment (CHT) (producing lattice defects as intragranular traps for phosphorus atoms) and optimizing the tempering process (balancing grain boundary segregation and lattice restoration). Whereas no effect of boron was determined, TMT reduces the sensitivity to phosphorus and leads to better ductility, especially for the highest strength Rm > 2000 MPa. The best strength ‐ ductility combination was found for the range of tempering temperatures ϑtemp between 280 and 330 °C even for phosphorus contaminated steels. Outside of this range of ϑtemp, there is a significant deterioration of ductility for P ≥ 0.02 %. Independent of P‐content, there is a dramatic decrease in 0.2 % proof strength for ϑtemp decreasing below 280 °C due to residual internal stresses.

Thermomechanical treatment to improve the fatigue behaviour of phosphorus containing high strength spring steels

“…Moreover, Augerelectron spectroscopy revealed that all variants with .9de£ .9def,2 < 800 DC produced a higher phosphorus enrichment at the grain boundaries than the optimum variant with .9def,2 = 800 DC which was finally applied for the following fatigue tests. that for a phosphorus concentration higher than approximately 0.02 % the ductility decreases with raising tempering temperature in the range &temp > 330 DC [7]. Moreover, for tempering temperatures in that range Z decreases with increasing phosphorus content above the threshold of P = 0.02 %, but maintains constant for P < 0.02 %.…”

“…This paper is focused on the possibilities to reduce the detrimental effect of phosphorus on the fatigue properties of CrY high strength steels for power leaf springs by optimizing process parameters and employing TMT. The results of the basic investigation on the static properties of phosphorus contaminated CrY steels are given in [7].…”

“…Some of the heats were micro alloyed with niobium (0.032-0.035 %), mostly for TMT variants, to meet the demands of the special manufacturing of leaf springs as described later on. Outgoing from results in [7], where the threshold value of 0.02 % P was found, above which the effect of phosphorus becomes effective, all alloys -without and with Nb -were divided in two groups concerning phosphorus content. The cast blocks of experimental heats have undergone a sufficient plastic deformation [8] and final normalizing at 950°C before the experimental processing to be comparable to the hot rolled strips from the industrial production.…”

“…Moreover, for tempering temperatures in that range Z decreases with increasing phosphorus content above the threshold of P = 0.02 %, but maintains constant for P < 0.02 %. These phenomenons can be explained in terms of time/temperature dependent and multi-eomponent segregation theory [7]. Based on this knowledge the Woehler curves are labelled with regard to tempering temperatures and phosphorus content (below and above 0.02 % P).…”

“…Comparison of the manufacturing process of a parabolic leaf spring by conventional heat treatment (a) and thermomechanical treatment (b)Though an extreme tensile strength of more than 2100 MPa is feasible by lowering the tempering temperature &temp, the ductility (Z value) and the 0.2 % proof strength decrease with decreasing &temp below 280 DC[7]. With 9temp of about 280 DC an excellent strength-ductility relation of R m '" 2100 MPa and Z", 40 % is achievable.The cyclical tests.…”