Frequently Asked Questions
Oxyacetylene welding is one of the most well established methods of joining cast iron. Use of a gas flame to heat the casting to its melting point is time consuming, and can lead to surface oxidation, but the high preheat of the base metal, and the low cooling rate which results, ensure that formation of hard phases such as martensite in the HAZ can be avoided. However, since a large volume of metal is heated, contraction strains are generally large and can produce distortion of the casting, leading to a loss of dimensional accuracy. The process is also very slow compared with arc welding. Where colour matching between weld and casting is important, oxyacetylene welding provides the most reliable way of depositing cast iron filler metals which are soft enough to be machined. There is also the added advantage of very low penetration and dilution when using this process.
Many of the disadvantages of fusion welding can be avoided by using bronze (or braze) welding, a process in which the parent material is not melted. Braze welding involves introducing a filler metal which melts at a lower temperature than the iron, e.g. alloys based on brass, which have liquidus temperatures of 800-940°C. The pick-up by the braze metal of impurity elements such as phosphorus and sulphur from the casting is minimised since the parent material is not melted. Likewise, the maximum heat affected zone temperature is much lower, and so hardened structures are not produced and the soft braze metal is generally easily machineable.
The disadvantages of the method are that the joint properties are controlled by a relatively soft braze metal or a rather weak braze/cast iron interface. The lower melting and softening temperature of the deposit compared with fusion welds also limits the maximum service temperature of braze welds, typically to around 260°C for a brass filler. There is also poor colour matching between the braze and the casting. In addition, it is more difficult to detect imperfections in braze welds than in fusion welds, because of the lack of the same degree of metallurgical continuity along the bond line as a whole. Another potential disadvantage of the process is that corrosion of the joints can be accelerated by galvanic effects arising from the difference in chemical composition between the casting and the deposit.
See also Welding of cast irons - a guide to best practice , from which this FAQ is extracted.