Failure of Hot-Rolled Steel Bars

Two 25 x 40 mm (1 x 1.5 in.) AISI 4150 hot-rolled steel bars that cracked during heat treatment were examined to determine whether the heat treating procedure had contributed to the failure. Metallographic examination of a cross section taken through the fracture revealed an oxide coating on both sides of the fracture surface. The oxide was also found on the top and bottom sides of the sample. Sawcut sides of the bar did not exhibit the oxide layer The presence of the oxide in the fracture, combined with its absence on all exterior surfaces, indicated that the fracture occurred as a result of an oxide seam in the original material rather than from oxide from heat treating. Nondestructive testing prior to machining and heat treatment was recommended.