Optimizing Microstructure and Mechanical Properties of Cast AISI-H11 Hot Work Tool Steel by Alloying with Niobium and Boron

Hot work tool steel like grade AISI-H11 is used extensively to manufacture dies for metal forming like die casting, forging and extrusion dies due to its good toughness to hardness ratio and good resistance to offset fatigue strength. However, there are many application drawbacks like prior austenite grain gross and surface cracking of dies after short service life time. New cast H11 steel containing niobium and boron as alloying elements has been designed in a trial for better structure stability, higher toughness and fatigue resistance as well as low production cost. The mechanism is to increase strength and toughness through grain refinement of the steel microstructure even after many thermal service cycles through alloying with such elements. Experimental cast hot work tool steel dies with different alloy additions were produced, machined, heat treated and then mounted into Al-Si die casting machine to evaluate the changes in microstructure and mechanical properties related to its thermal life cycles. Universal testing machines to detect hardness, Charpy V-notch impact toughness and bending fatigue together with metallurgical microscopy, image analysis microscopy, SEM=EDX and X-ray diffraction techniques were used to measure the mechanical and microstructure properties of the produced steel alloys. The results showed improvement in microstructure grain refinement and mechanical properties of H11 cast hot work steel dies when alloying it's melt with 0.3% B + 0.4% Nb. Improved hardness (48 HRC) and toughness (29.5 J) together with more bending fatigue life time to reach one million (1 M) cycles when applying mean stress at 750 MPa were obtained for such cast modified steel alloy