A.Mechanical performance characteristics of gray cast iron
1.Common gray cast iron due to the presence of graphite,and graphite tensile strength is almost zero,can be cast as full of cracks or hollow steel.Not only undermine the continuity of the graphite matrix,reducing the load-bearing metal matrix effective cross-sectional area,so that the actual stress is greatly increased;at the same time,the graphite is easy to cause stress concentration at sharp corners,sharp corners of the stress is much larger than the average stress.Reduce the role of the former is called graphite,which is called graphite cutting action.Therefore,the gray cast iron tensile strength and elastic modulus than steel is much lower,generallyσb is about 120~250MPa,compressive strength and steel close,generally up to 600~800MPa,ductility and toughness is close to zero,is brittle material.The greater the number of gray cast iron in the graphite sheet,the larger the size,the distribution is uneven,impact on the mechanical properties will be.But the presence of graphite has little influence on the compressive strength of gray cast iron,gray cast iron because of the compressive strength depends on the matrix organization,compressive strength similar to steel gray cast iron.When there is a similar sample of gray cast iron in the graphite sheet as sharp notch,notch in stress close up to more than five times the average.The presence of such stress concentration,so that even in the gray cast iron to withstand a relatively small load(far reaches the yield strength of the matrix),the actual stress at the edges of the graphite matrix will exceed its yield strength,so here there will be a residual metal deformation,even cracks(when the actual stress exceeds the ultimate strength of the substrate).When this graphite edge crack,further reducing the gray cast iron to withstand the load of the effective cross-sectional area,and is exacerbated by the phenomenon of stress concentration,stress concentration to the top of the crack along with the rapidly moving so quickly crack extension,the entire cast of brittle fracture occurs.Therefore,due to the presence of graphite cut and cutting effects caused by the strength of the cast iron metal substrate can not fully play,according to statistics the utilization of ordinary gray cast iron matrix strength of generally not more than 30%to 50%,which showed gray cast iron tensile low intensity.In addition,severe stress due to the presence of graphite concentration phenomenon caused by the early cracks caused by the occurrence of resistance to crack development and poor,leading to brittle fracture,so the gray cast iron plasticity and toughness almost not be manifested.Obviously,due to the performance degradation caused by the presence of flake graphite,the total effect is not the algebraic sum of the substrate cutting action tends to harm both much stronger than the reduction effect.Incidentally,ordinary gray cast iron when under stress,at the edge of the graphite due to stress concentration often cause a small amount of residual strain.Thus,the stress-strain curve of gray cast iron even under low stress is not linear,but there is a certain curvature.Therefore,the elastic modulus of gray iron only relative significance.
II.Hardness characteristics of gray cast iron
In steel,Brinell hardness and tensile strength of the relatively constant,approximately equal to 3,in cast iron,this ratio is very dispersed.When the same hardness,have a tensile strength range.Similarly,when the same intensity,there is a range of hardness,it is because the strength properties of the graphite influenced by,while hardness reflects substantially only due to the base case.Many factories of cast iron hardness estimate its tensile strength,a lot of information also raised the relationship betweenσb and HBS.It must be noted that this estimate is only stable under the process conditions,the basic parameters of the graphite sheet is reliably close the case.Depends on the hardness of gray cast iron matrix,which is due to the determination of the hardness of the ball is pressed against the test piece,the size of the ball with respect to crack graphite is a rather large,so the main force to bear on the substrate,and therefore with the the increase in the number of pearlite matrix,dispersion becomes large,the hardness is improved accordingly,when the metal matrix occurs when the hard constituent phases(such as free cementite eutectic phosphorus,etc.),the hardness increased.
3.All other performance characteristics of gray cast iron
While reducing the mechanical properties of graphite gray cast iron,gray cast iron but to bring a range of other excellent performance.
① good casting performance when gray cast iron casting forming,not only their mobility,but also because of precipitation during solidification hematocrit larger graphite,reduced solidification shrinkage,easy access to an excellent cast,showing good casting properties.
② good damping properties of the graphite cast iron can withstand vibration dampening effect,weakening the vibration energy transfer between the grains,and the vibration energy into heat energy,so the gray cast iron has good damping properties.
③ good wear resistance of graphite itself is a good lubricant,graphite off the friction surface can act as a lubricant,which has good abrasion resistance gray cast iron.
④ good cutting performance during cutting,anti-friction graphite plays the role of chip breaking.Since the graphite off the formation of microscopic pockets,from the reservoir effect,it can maintain the continuity of the film,so the good performance of gray cast iron machining,tool
⑤ low notch sensitivity flake graphite is equivalent to many small gap,thereby reducing the sensitivity of the casting gap,so that the surface quality is not high or tissue defects on the fatigue strength of cast iron adverse effects than too little impact on steel many.
IV.Classification and application of gray cast iron
In the actual production of cast iron is often put forward various demands,since the use of various pieces of cast iron vary,also suffered a load sizes.Therefore it is proposed to cast different performance requirements,which is to draw up the actual basis of cast iron grades.
Strength properties of cast iron is one of its most important quality indicators,so the gray cast iron grades are its tensile strength to the division.Grades of gray cast iron HT×××,where"HT"stands for"gray iron"pinyin word prefix,while the back×××minimum tensile strength value,the unit is MPa.Total of six grades of gray cast iron,which HT100,HT150,HT200 ordinary gray cast iron,HT250,HT300,HT350 cast iron as inoculation.According to different grades of gray cast iron and Optional:low load and unimportant parts such as shields,small handle,cover and weight,etc.,you can use an ordinary gray cast iron(eg HT100 or HT150);withstand medium load of parts,such as base,frame,box,pulleys,bearing housings,flanges,pump,valve,pipeline,the flywheel and the motor mount,etc.,generally used in the breeding of cast iron(eg HT250);bear a greater burden of the important parts,such as base,bed,gears,cylinder,flywheel,gear boxes,medium pressure valve,the cylinder block and cylinder liners,etc.,to use more high-strength HT250 gray cast iron;withstand high loads,requiring high wear resistance and air tightness the important parts,such as heavy machine bed,machine body pressure,high pressure hydraulic parts,piston rings,gears and cams,etc.,generally used in the above high-strength HT250 gray cast iron.