A Discussion of the Strength and Toughness of Secondary Hardening Steels of Moderate Alloy Content

Hot work die steels such as H-11 and H-13 can achieve strength levels comparable to those of 4340 and 300M tempered at low temperatures.  However, steels such as H-11 and H-13 are not used in toughness critical applications because when these steels are tempered to have high strength they have fairly low fracture toughness.  This talk will discuss the effects of the non-carbide forming alloying elements, aluminum, silicon, cobalt and nickel on the strength and toughness of medium carbon hot work die steels.  All of these elements can significantly enhance the secondary hardening response of these steels.  Of these silicon has the strongest effect on the tempering response and 2 wt. % silicon is more effective in enhancing the secondary hardening response than 8 wt. % cobalt.  Silicon promotes the formation of undissolved carbides so if high silicon additions are to be used to enhance the tempering response higher austenitizing temperatures should be used.  After austenitizing at 1050°C, the alloys modified with additions of 2 wt. % silicon, 2 wt. % aluminum and 8 wt. % cobalt have peak secondary hardnessess of about Rockwell-C 57 but also have low toughness as measured by the Charpy impact energy.  The Charpy impact energies at peak secondary hardness were 16.3 J, 14.9 J and 6.1 J for the alloys modified with aluminum, silicon and cobalt, respectively.  An addition of 4 wt. % nickel increased the peak secondary hardness by about 2.5 points on the Rockwell-C scale and and the nickel modified heat had a Charpy impact energy of 32.3 J at a Rockwell-C hardness of 54.3.  Based on these results a new experimental heat was made.  The fracture toughness of this new heat was surprisingly high. This new heat can achieve a fracture toughness of 145 MPa√m at a yield strength of 1585 MPa and an ultimate tensile strength of 1882 MPa.