COLD-FORGING FOR HIGH-STRENGTH LOWER COST STEEL FASTENERS

High tensile steel fasteners, up to about 30 mm diameter, are currentl y cold formed from bar, rod, or wire. However, cold forming is largely restricted to upsetting the head, usually with very little work in th e shank. Present fastener standards then require the strength to be in troduced by hardening and tempering. They also specify the level of ha rdness to be obtained in the as quenched condition, and a minimum temp ering temperature. Consequently, the steels need to have sufficient al loy content to ensure both effective hardening response and temper res istance. Thus, the overall cost of fastener manufacture includes contr ibutions due to steel composition, any annealing which may be required to soften the bar, rod, or wire, and the final heat treatment to ensu re the required properties. This research was carried out to assess th e potential for reducing the manufacturing costs for high tensile fast eners. The aim was to obtain 8.8 fastener properties (as specified in ISO 898/1) from as rolled low carbon ferrite-pearlite steels strengthe ned by cold work, thus minimising the alloy and rod processing costs i n addition to eliminating the cost of the hardening heat treatment. As rolled rods, with a ferrite-pearlite structure, were produced for eva luation. The steels had variations in the pearlite content, controlled by manganese content, while additional steels contained higher silico n or a microalloying addition of niobium. Initially, the hardness of d rawn and tempered rod was determined. From the results obtained, steel s with selected combinations of cold strain and tempering were subject ed to an additional evaluation of mechanical properties at ambient, su bzero, and elevated temperatures. The ambient temperature properties r equired for 8.8 fasteners were attainable in the C-Mn steels containin g over 18% pearlite and more readily in the silicon alloyed variants a fter drawing and tempering at temperatures in excess of 450 degrees C. However, the best combination of properties was obtained in the niobi um microalloyed steel producing 8.8 properties, with an impact transit ion temperature below -60 degrees C, after drawing by 45% reduction an d tempering at 450 degrees C. The cold worked and tempered steels also showed better stress relaxation characteristics than conventional har dened and tempered steels. Subsequently, fasteners were made from the high silicon and microalloyed steels using a commercial cold forming m achine. The total strain used was introduced by various combinations o f drawing, extrusion, and heading. After tempering, the fasteners prod uced the same tensile strengths as had been obtained in rod drawn by t he same overall strain. This research has shown that fasteners meeting the properties specified in standards can be produced in as rolled fe rrite-pearlite steels by utilising strengthening from cold work, Adequ ate properties were achieved in cold worked and tempered low carbon ma nganese steels, but microalloying with niobium produced the optimum co mbination of properties. Using the steels and processes described offe rs a potentially cheaper alternative to heat treatable steels for the manufacture of high strength fasteners.