TRC Test Results of Various High Strength Steel Welds and Effects of Hydrogen

A Study of Root Cracking in High Strength Steel Welds (Report 5)

Abstract

The effects of restraining force on root cracking of various high strength steel welds, ranging rom 50 to 80 kg/mm², were investigated by the NRIM TRC test (Tensile Restraint Cracking test). Crittical transverse tensile stress, which is necessary to initiate a root crack in the first layer of a weld, was determined for each weld under several welding conditions. Effects of cooling process below 300°C, preheating temperature and hydrogen content on critical stress were discussed. The conclusions obtained in the study are summarized as follows:
(1) The behavior of root cracking in the TRC tests on high strength steel welds is a delayed failure type and practically identical to that in the slotted groove restraint cracking tests
(2) From the TRC tests of various high strength steels of tensile strength grades of HT 50 to HT 80, the values of critical tensile stresses were obtained below which no root cracking occurred.
(3) The value of critical tensile stress is generally increased with an increase of weld heat input or preheating temperature and a decrease of diffusible hydrogen content. The value is generally greater, the lower is the tensile strength of the steel.
(4) The welding conditions and preheating temperatiure for which the TRC critical tensile stress is approximately equal to the yield stress of weld metal, are satisfactory to prevent root cracking in slotted groove restraint cracking tests.
(5) The cooling process below 300°C has considerable effect on the value of critical tensile stress of HT 80 steel welds, but liftle on HT 60 steel welds. Faster cooling is harmful.
(6) The correlation among the value of critical tensils stress σc, (kg/mm²), preheating temperature Τ₀ (°C) and diffusible hydrogen content [Η] (cc/100 gr) of an HT 80 steel 8 E weld is summarized by the following experimental formula :
logσ_c=1.845-(200-Τ₀){0.02log([Η]+1)-0.003}
(7) The correlation, like as above formula, in an HT 60 steel 6 F weld is summarized by the following experimental formula :
logσ_c=1.64-0.156log([Η]+1)+0.001Τ₀
(8) A little content of diffusible hydrogen is detrimental to root cracking in HT 80 welds, but not in HT 60 welds, and preheating is much more effective for the prevention of root cracking in HT 80 welds than in HT 60 welds.