An attempt has been made to study the effects of molybdenum and nickel on the structure and properties of 0.11% carbon steel. Although research has been done on low carbon molybdenum steels, but almost no detailed research is available on the effect of a small addition of molybdenum on the structure and properties of low carbon structural steels, current studies have shown that molybdenum is the grain size improves and influences the transformation properties and properties of carburized steels. The current work therefore; will investigate the effect of a small addition of molybdenum lind-nickel individually and in combination on the structure and properties of low carbon structural steels.
HIGH STRENGTH, LOW ALLOY (HSLA) STRUCTURAL STEELS
Requirements
Factors Affecting Strength and Toughness of HSLA Steels
- Ferrite Grain Size
Grain growth involves an increase in grain size with a corresponding decrease in grain number. Orwan1S showed that the yield strength of the alloy is the sum of the matrix flow stress rOland the shear stress resolved in the arc dislocation between the dispersed particles, i.e. conditions are met.
Effect of Cooling Rate on Grain Size in a Carbon Steel
Phosphorus is an effective strengthening agent in steel, but too high a high phosphorus content can be harmful to the properties of the product, especially content above 0.1-02%. The strengthening effect of the carbon is relatively small and depends significantly on the grain size20. the effect of sulfur is striking, as it is the only element that reduces the annealed steel; a similar effect has been observed by Gladman et al. This calculation shows that the number of new grains is a function of the ratio of In/G. Since the grain size of a new phase is proportional to n'l.'.', the grain size of a new phase is a function of the ratio ltJG, When ratlo ItiG increases ""with a decrease in the transformation temperature, the grain will si7e of a isothermal transformation temperature.
In a transformation involving continuous cooling, the transformation occurs within a specific temperature range and this range decreases as the cooling rate increases. Thus, for a transformation where the ratio of nucleation rate to growth increases with decreasing temperature, the grain size of the new phase decreases with increasing cooling rate. This is the reason why fern grains become refined with accelerated cooling. The continuous cooling transformation can b" be considered as the sum of short-term isothermal retention; al ;ucceSSl,e t",mp",ralure. However, since ferrite nucleation and ferrite growth rates are only a function of the current temperature, Based on this assumption, the number of ferrite grains nucleated at temperature T, during cooling, given as.
The ferrite grain size is proportional to d,"l when austenite grain surfaces are the dominant nueleatlon site for ferrite. Thus the theoretically estimated fernte b'Tain size can be expressed as a function of cooling and gram Slze as. The effect of increasing the cooling rate is to increase the strength, decrease the ductility and decrease the toughness. The increase in strength occurs as a result of a decrease in the proportion of ferrite and an increase in the amount of others (for example, pearlite or bamit}lo,.
Ferrite Grain Size Dependence on Austenite Grain Size
The effect of increasing cooling rate is to increase the strength, decrease the ductility and decrease the toughness. The increase in strength occurs as a result of a decrease in proportion of ferrite and an increase in the amount of others (e.g. pearlite or bamite}lo, . site density is proportional to d/ since the grain surface per unit volume is proportional to d; l. When ferrite nucleation occurs at austenite grain boundaries, nucleation site density is proportional to d,-2, smce the grain edge length per wtil volume is proportional to ,V. Thus, ferrite b'Tainsize is proportional to d,11J When ferrite nucleation occurs at austenite gram angles , nucleation site is denslly proportional to d,-) since the number of grains per umt volume is prOlXlrtionalto d,.-J Thus, fernte gram size is ProlXlrtionalto d,. In summary, the dependence of ferrite grain size on austenite grain size is as follows' d/'" for grain surface nucleation, d,.IIl for b'Tainedge nucleation, and d, for grain corner nucleation For homogeneous nucleation, femte b'Tamsize is independent of austenite gram size.
In an actual ferrite transformation, several types of nucleation sites operate simultaneously, and the dependence of fernte b'Tainsize on the austenite grain size is a weighted average of different nucleation modes.
Metallurgical Design of Ferrite.Pearlite steels
There are four factors that affect the formability of ferne-pearlite phases, namely: yield stress, work hardening and the total ductility before plastic instability, the first two properties must be low and the last two must be high. Pearhte and refinement of the interlamellar spacing of pearlite increase both the l10w stress and work hardening rale their decrease ductilitJi17. Refinemg the ferrite grain size cannot offset the disadvantageous elTeet of pearlite, but grain refinement is beneficial in tens of total ductility at fracture. Non-metallic indusions reduce the total malleability extended indllSies are particularly detrimental to transverse ductilit} and energy of the upper part of the back, Sib'llIfieant impro,ements in transverse duetihty and upper shelf ener!:,'ybaat made in botb plate and strip by reducing the volume fraction of inclusion, or by inclusion shape control Induction shape is modified by adding elements such as titamum, zirconium or rare earth to replace the elongated inclusions by spberic one which can reduce the length ductility slightly, but significant increases in botb long - transverse and through-thickness ductility occurs.
Therefore, the carbon and sulfur content should be 10", and the non-metallic inclusions should be informally distributed and shape controlled to achieve good fonnabiHt},.
Materials Preparation
Measurement of Prior Austenite Grains
This technique is based on the formation of a complete or almost complete network of ferrite or iron carbide in hypoeutectoid or supereutectoid steel. 34;,'with a carbon content far from the eutectoid composition, after slow cooling through the transfOrmalQn zone, will reject either ferrite or cementite, depending on the carbon content of the steel. Rejection of these nccun ingredients; mostly at the boundaries of the previous austenite grains and under the right conditions the constituents can form to form a network around the original grains. The efficiency of this process is NOT good for alloy steel containing Ni and Mo, because at Isotennaltemperature fine fernte b'Iains would form Illstead Qf cQmpletc or almost perfect ferrite nctwork, As a result carburization technique was followed to reveal the austenitic grain boundaries.
This technique is based on the formation of a continuous cementite network at the austemte grain bQWldaries. Carbon will diffuse into steel forming cementite at the austenite boundaries during austenitizing temperature. The steel samples were heated to different austenitizing temperature, i.e. Before these samples are heated, they are packaged.
They were then placed in the Blue-M oven and the oven was turned on. After the desired temperature was reached, they were held at it. temperatures for 1 hour and then cooled in this oven to room temperature. The grain size was determined using the linear intercept method, where the grain boundary and the intersections with the circumference of the circle were counted in the eye of a microscope. The elective circumference of the circle was accurately determined by measuring the dm-meters" with reference to a stage micrometer at the magnification used. In total, at least 600 intersection points were counted for each specimen.
The overburring tcchnique was found troublesome m revealmg the previous austenite grain f. limits for steels containing Ni and much time "as spent in obtaining satisfactory) rewlts.
Determination of Heat-treatment Temperature
Heat-treatment of Specimens
Preparation and Mechanical Testing of Tensile Specimens
Optical Microscopy
Determination of Ferrite Grain Size and Hall-Petch Plot
These measurements showed that although all steels started out with the same austenite grain size, their subsequent ferrite grain size at a given cooling rate was different. A Hall-Petch plot of yield strength versus d-l was drawn for each sample to get an idea of how yield strength varied with ferrite grain size.
RESULTS
Prior Austenite Grain Size
Heat-Treabnent Temperature
Optical Microscopy
The mechanical testing data together with the volume fraction of pearlite and true ferritic grain size of the steels are presented in Tables 7.10. The steels initially had a ••••total austenite grain size, but their subsequent ferrite size in grams under the same cooling conditions was not the same. This indicated that the precipitation kinetics and effectiveness in controlling the ferrite grain size of MOle and other factors are different.
CHAPTERS
DISCUSSIONS
- Prior-Austenite Grain Size
- Carburization Technique
- Effect of Precipitates on the Prior Austenite Grain Sizc
- Effect of precipitate on Ferrite Grain Size
- The Hall-Petch Plot of the Steels
- Yield Strength Increment from Precipitation Strengthening
Therefore, this technique for measuring austenite grain size was rejected and the carburizing technique was adopted for this purpose. So this grain size was measured by trial and error. The effectiveness of this procedure for steels 1-4 in revealing the preliminary austenitic grain boundaries will be seen in Figure 2. It can be seen from Figure 3 that the austenitic grain size of steel 1 increases linearly and rapidly with increasing temperature in the investigated temperature range.
It is clear from Figure 3 that the austenite grain size of steel 14 increases almost and rapidly as that of steel 1. Second phase particles in the other steels clamp the austenite grain boundaries, causing the steels to maintain a fine grain size, which remains virtually unchanged during the process. heating at successive high temperatures, once the grain growth inhibitors prove ineffective at some elevated temperatures due to solullon or coalescence, the grain coarsening rate is greater than that of city I This is due to the fact that "when the precipitation is no longer effective, the limited grains want to reach equilibrium so that rainfall increases as quickly as possible, thus causing an abrupt growth in precipitation." grain size,. The average fermente grain size of the steels at four different cooling rates used in the present experimental work is sho.
Coarse graining of fernia steel stcel 2 than plain carbon steel I clearly indicates that MOle documents do not produce any refinement of fernia grain. Neglecting the Widmanstatten ferrite-pearlite, it was possible to estimate the ferrite grain size of the steel under these conditions by taking only the fields showing the polygonal ferrite-pearlite structure, Steel 4 also contains the Wldmanstatten ferrite-pearlite structure at a cooling rate of 120 "C /min.The ferrite grain size of this steel was also estimated in the same way.This procedure may introduce some errors in the ferrite grain size measurement at these cooling rates.
The results are obtained by subtracting the yield strength of the base steel at the samc grain size from the observed yield strength of any other stec1 at caeh cooling rate,.
CONCLUSIONS
Conclusions Drawn from the Present Work
As the cooling rate increases, the increase in yield point (t.YS) due to M02C precipitates increases. ix). In the presence of nickel, the contribution of MolC to the yield point is greater than that of Mo2C in the absence of nickel.
Suggestions for Further Work
Petch,NJ, 1953,HSJ, 174,25
120 C Imin
