| Abstract: | Laboratory melting of low carbon boron-molybdenum steels was successivelycarried out in high frequency furnace.It was found that the steel after forging andnormalizing treatment,a maximum stress of more than 60 kg/mm~2 and a yield stressof more than 48 kg/mm~2 can be obtained.The value of yield stress is more thandouble the yield stress obtained from unalloyed low carbon construction steels andthe steel at the same time retaining good ductility and toughness.The welding pro-perry of the boron-molybdenum steel was found to be good and this steel is recom-mended to be used as a high strength low alloy construction steel.The effect of boron in increasing the mechanical strength of 0.50% molybde-num steel was found to be due to the alteration of the carbide and ferrite dispersionin the microstructure after the normalizing treatment.High maximum and yieldstresses can be obtained from a microstructure consisting of a mixture of fine disper-sed pearlite and acicular ferrite,and the microstructure of the boron-molybdenumsteel is markedly influenced by the normalizing temperatures and cooling rates.By studying the effect of boron on the decomposition of austenite,it was found thatboron greatly retarded the formation of proeutectoid ferrite and pearlite,thus in-creasing the hardenability of the steel.Thus even at relatively slow cooling rates,a high mechanical strength can be still obtained from the boron-molybdenum steel.Calculation according to empirical formulae from the isothermal transformationdiagram of the boron-molybdenum steel shows that at a normal air cooling rate,the maximum diameter of the steel bar which can be used to obtain high me-chanical strength must not exceed 75.2 mm. |
