Welding Process for Ductile Cast Iron and Low-Carbon Steel

Here, we introduce the welding processes for ductile iron and low-carbon steels No. 20 steel and No. 15 steel.

(1) Automobile Drive Shaft Welding

A typical product for automotive drive shafts is made by welding ductile iron QT400-15 and 20 steel tubes together using an arc cold-welding process. The key technical points of the arc cold-welding process are as follows:

Before welding, clean the weld joints of ductile iron QT400-15 and 20 steel pipes to remove oil stains and rust, exposing the metallic luster. The cleaning area around the joint should extend 20–30 mm into the groove and on both sides.

Using the high-plutonium welding electrode EZV (Z116) as filler material ensures no cracking after welding. When selecting structural steel welding electrodes, E5015 or E5016 (J507 or J506) may result in some cracks on the weld, but the mechanical properties of the repaired weld joint still meet the process requirements.

When welding, use a small current, low voltage, and short-arc technique—weld one layer for the small transmission shaft, and two layers for the large transmission shaft. After welding the first layer, temporarily cool the area before proceeding to the second layer, to prevent overheating in the connector region.

After welding, allow for slow cooling to prevent cracks caused by the differing linear expansion coefficients of the two base materials. In mass production of transmission shafts, post-weld slow cooling can be carried out in a heat treatment furnace.

(2) QT400-15 and 15 Steel Carbon Dioxide Gas-Shielded Welding

Welding of ductile cast iron QT400-15 and 15 steel can be performed using CO2 gas-shielded welding. When using Ho8Mn2SiA welding wire as the filler material for the joint, it is first necessary to apply a high-vanadium transition layer onto the beveled surface of the QT400-15 ductile iron. This high-vanadium transition layer effectively eliminates the white structure, enabling the tensile strength of the weld joint to reach 431–441 MPa. Alternatively, when selecting a high-performance flux-cored wire, the tensile strength of the welded joint can achieve 402–431 MPa.

Practice has proven that when using CO2 gas-shielded welding for ductile iron and carbon steel, welders can achieve rapid melting, deep penetration, minimal weld fusion ratio, convenient adjustment of welding parameters, and precise control over both welding heat input and weld cross-sectional dimensions. Meanwhile, the CO2 gas flow provides a cooling effect on the weld and heat-affected zone, helping to reduce thermal stresses in the welded joint and minimize the width of the heat-affected zone. This, in turn, effectively prevents welding cracks and enhances the machinability of the welded joint. As a result, this method has gradually been accepted and adopted by welders.

Ductile iron undergoes spheroidization and specialized processing to produce a material with spherical graphite, effectively enhancing its mechanical properties—particularly improving its ductility and flexibility—while achieving compressive strength that surpasses even that of high-carbon steel.

In China, the typical spheroidal graphite cast iron components usually require a sphericity grade of Level 4 or higher, meaning a sphericity rate of at least 75%, while standard components generally achieve around 86%. In recent years, with the rapid growth of spheroidal graphite cast iron production and processing—particularly in industries such as wind power generation casting and sectors with stringent quality demands—the required grade for spheroidal graphite cast iron has risen to Level 2 or above, enabling bearing ball yields of over 95%. Through continuous analysis and refinement of the QT400-15 spheroidization process, along with optimization of spheroidizing agents and inoculants, the sphericity rate of spheroidal graphite cast iron has now surpassed 95%.