There are a lot of factors as following, which make the workpiece easy or hard to be machined.
1)Physical Performance
a)Thermal conductivity K,ther higher thermal conductivity the higher VC.The VC with tungsten carbide cutting tools as below
Carbon steel: K=48.2～50.2 W/m?k Vc=100～150 m/min
High termperature alloy: K=8.4～16.7 W/m?k Vc= 7～60 m/min
Titanium alloy: K=6.3～9.6 W/m?k Vc= 15～50 m/min
b)Coefficient of linear expansion,it influence the extent of material hot-expansion but cold-shrink performance,leading to the precison influenced.
2)Chemical content
The chemical content and proportion take role in material mechanical property,physical performance,heat treament performance,metallurgical structure and machining
capability.For example
a)The higher carbon content,the higher hardness and toughness.
b)Ni is available to improve the heat toleration of the material,but reduce the thermal conductivity significantly,when the Ni>8%,it makes austenitic steel and hardness improved.
c)V,with the increase more highly of V,the grinding performance is worse.
d)Mo,it is available to improve the material strength and toughness,but reduce its thermal conductivity.
e)W,it helps to improve the material's hot and high temperature toughness,but also make the thermal conductivity decrease dramtically.
f)Mn,it could increase the material's hardness and strength,but decrease its toughness.When Mn>1.5%,its cutting performance becomes badly.
g)Si,it makes the thermal conductivity low.
h)Ti,it is an element easy to be carbided,but the machining capability is also not good.
3)The materials' merchanical performance.
a)Hardness and toughness,the material's hardness and toughenss is medium,which has good machining capability,but if the hardness and toughness is very high,the machining capability reversed.Such as normalized 45# steel hardness HB200,σb 64;hardened 45# steel,hardness HRC45,σb 2100～2600 MPa.
b)The fine debris existed in the material also influece the machining capability,like A1203、Si02、Ti02 etc,which have very high micro-hardness,resulted in mechanical wear on the cutting tools and low machining performance.
c)Toughness αk and plasticityδ,the material with high toughness and plasticity,means the high resistance,deformation and heat in the machining process,leading to bad machining performance.
d)Modulus of elasticity E,it is an index of material rigidity,if the material has highmodulus of elasticity,it would not be elasticity deformation under the outside force,but the low modulus of elasticity material,the elasticity is restored quickly,leading to high fray with the cutting tools and hard to be machined.Such as soft rubber E=2～4MP,45# steel E=200000 MPa,Mo material E=500000 MPa.
4)Microstructure.
a)Ferrite,it has very low hardness and strength(HB50～90，σb =190～250 MPa),toughness and plasticity is high(δ=40～50%),leading to built-up edge in the cutting process and very had machining capability.
b)Pearlite,sopheric pearlite with very good machining performance,such as 45# steel.
c)Carburized mateiral,high hardness(HRC66-70),but very fragile(αk=30-35 MPa),as its edge is very easy to be broken owing to the incease of Fc,resulted in bad machining performance.
d)Austenite,its hardness is not high(around HB200),but with high toughness and plasticity ,surface hardened and built-up edge and cold-welding with the cutting tools,resulted in bad machining performance.
e)Martensite,hardened steels belong to this category,which has high hardness and fragile,only account for 1/3～1/10 of 45# steel's machining performance
