An important Factor for Leaf Spring Manufacture Raw material for Leaf Springs
The basic requirement for spring steel is that it should have sufficient hardening ability relative to leaf thickness to ensure a fully martensltic structure throughout the entire cross section of the leaf spring. Leaf springs are made of various fine grade alloy steel. The most commonly used grades of spring steel are 55 Si 7, 60 Si Cr 7, 50 Cr V4. We at UAW manufacture springs using EN 45A, 55 Si 7, 60 Si 7, 65 Si 7, 55 Si Cr 7, 60 Si Cr 7 & 65 Si Cr 7 grades of steel, however we offer other grades based on customer requirement. The spring steel flats should be reasonably straight and straightness to the extent of 2 mm per meter length is allowed. The flats should be free of defects like Piping, Seams, Edge Cracks, End Kinks, Rust pitting and other Rolling Defects. Flats shall usually be with round edges. The edges shall be rolled convex with the radius of curvature of the edge approximately equal to the thickness of the flat or as agreed between the purchaser and supplier. Different cross sections of steel are used for the manufacture of leaf springs depending on the design. At UAW we use about 40 cross section of steel for manufacture of springs for domestic & export market. The tolerances for cross section of spring steel as specified in IS:3431 1982 is given in Table 2. The chemical composition of spring steel mentioned above is as under.
|1||EN 45A||0.55-0.65||1.70-2.10||0.70-1.00||0.040 Max||0.040 Max||-||-|
|2||55 Si7||0.55-0.6||1.50-1.80||0.70.1.00||0.045 Max||0.045 Max||-||-|
|3||60 Si7||0.55-0.65||1.50-2.00||0.80-1.00||0.040 Max||0.040 Max||-||-|
|4||65 Si7||0.60-0.68||1.50-1.80||0.70-1.00||0.050 Max||0.050 Max||-||-|
|5||55 Si Cr7||0.50-0.60||0.70-1.00||0.50-1.80||0.025 Max||0.025 Max||0.20-0.40||-|
|6||60 Si Cr7||0.57-0.65||1.50-1.80||0.70-1.00||0.030 Max||0.030 Max||0.20-0.40||-|
|7||65 Si Cr7||0.60-0.68||1.50-1.80||0.0-1.00||0.050 Max||0.050 Max||0.20-0.40||-|
|8||50 Cr V4||0.47-0.55||0.15-0.40||0.70-1.00||0.035 Max||0.035 Max||0.90-1.20||0.10-020|
|WIDTH||WIDTH||Upto 10 mm||Over 10 mm|
|40 up to 50 mm||+/-0.30||+/-0.15||-|
|over 50 to 75 mm||+/-0.50||+/-0.15||+/-0.20|
|over 75 to 100 mm||+/-0.70||+/-0.20||+/-0.25|
|over 100 to 125 mm||+/-1.20||+/-0.25||+/-0.40|
|over 125 to 150 mm||+/-1.20||+/-0.30||+/-0.50|
If silicon in the raw material is above 2%, the spring steel will show a tendency for surface decarburisation, even during the most scientific heat treatment.
It is necessary to maintain the carbon level of the spring steel within 0.55% to 0.65% to facilitate proper heat treatment. At present many spring manufacturers tend to produce raw material with low carbon to avoid the possibility of brittle failure, which occur due to improper heat treatment. Here it is needless to say that the springs made from steel with low carbon will have low fatigue life as the hardness of the material is below specifications.
Steel with phosphorus content of more than 0.24% and sulphur content of more than 0.016% is unfit to be used in the production of Leaf springs as they form phosphates and sulphites which segregate in billet form and considerably reduce the fatigue life of the Springs. UAW receives on an average of about 800-1000 M Tons of springs steel flats per month. On receipt of the raw material the QC department ensures that the Chemical Composition is as per the norms specified in the Test Certificates issued by the supplier. This is done by drawing samples from each load of material and the same is tested in accordance with the sampling plans. This mechanism ensures that the material is not used unless inspected. The material is inspected for –