Welding of cast iron.
Cast irons are high carbon materials, (relative to steels) with typically 2 to 4% carbon. At the temperatures involved in welding, the liquid metal in the weld pool becomes saturated with carbon and can then transform on cooling to hard metallic phases which are brittle and very difficult to machine. There is also a greater tendency for weld cracks in the heat affected zone (HAZ), particularly in the pearlitic grades due to the martensitic structure resulting from the rapid cooling. So welding must be carried out with the correct selection of process, parameters and materials, (with the exception of abrasion resistant cast irons (white cast irons) which are not weldable). Ductile (SG) cast irons are generally considered to be easier to weld than grey cast irons.
So most cast irons may be welded, which is carried out to join castings to other components in an assembly or to repair defects in a casting to make it fit for purpose. A pre-condition is that the welding is carried out professionally using the appropriate process and by appropriately trained personnel.
Here a new technical report, Cast Irons: Welding (PD ISO/TR 10809-2:2011)(1) is described which is intended “to assist the design engineer to understand and to acquire knowledge of how the family of cast iron materials can be welded and to utilise this technology to its full advantage in selecting the most appropriate technique for a particular cast iron.”
The report gives information on the production of welded joints and on welding for the repair of iron castings. A wide range of processes can successfully be used to weld cast irons which are described in the report:
- oxy-acetylene gas welding
- arc welding including gas shielded metal arc welding and submerged arc welding
- plasma arc welding with or without filler metal
- electron beam welding
- pressure welding processes (flash welding, magnetically impelled arc welding and friction welding)
- other welding processes – including:
(a) cast welding: welding by pouring liquid metal into a specially prepared groove in a casting
(b) liquid metal welding: welding with additional use of a metal arc welding process
Welding procedures and materials
Welding procedures are described in some detail including welding with:
- homogeneous filler metal
- semi-homogeneous filler metal
- non-homogeneous filler metal and
- without filler.
As previously described, the high carbon content of cast irons means the formation of these problems can be avoided through appropriate choice of welding process, process parameters and welding rod material.
As the report describes, the following major welding parameters/control variables are available.
a) Pre-heat temperature. To avoid martensite formation, the weld area should be pre-heated to temperatures above the start temperature of martensite transformation. Pre-heating will not prevent the formation of ledeburite.
b) Heat input should be as low as possible during welding.
c) Welding speed will vary depending on the welding procedure applied and the chemical composition of the cast iron type and grade.
d) Cooling curve. In principle, the required cooling curve can be determined from the time-transition temperature (TTT) diagram relevant to the cast iron material. For instance, continuously controlled cooling according to the appropriate TTT diagram can prevent the formation of martensite, e.g. in a flash welding machine. When manual welding methods are used, the cooling rate is influenced by the selected pre-heating temperature.
e) Welding procedure/welding parameter for automated procedures.
f) Filler metal. When manual or mechanised welding-arc processes are used, the filler metal is matched against the requirements of the weld or welded joint. This depends on whether the welding process is carried out with homogeneous, semi-homogeneous or non-homogeneous filler metal. No filler metals are needed for the pressure-welding processes described later in the text.
g) Post-weld heat treatment. Measures can be undertaken to remove undesirable structures, such as:
- martensite which can be removed by a sub-critical anneal (tempering);
- ledeburite which can only be removed by a graphitisation anneal at austenitising temperature.
However, these control parameters are not independent of each other and have to be coordinated with the welding procedure applied. (1)
Usefully the report describes both finishing and joint welding for the main different types of cast irons including:
- spheroidal graphite (ductile) cast iron – this is the material for which there is more experience of both joint and repair welding so more information is known
- grey cast iron, finishing and repair welding only; the report states that there here is little experience of joint welding
- compacted graphite cast irons - repair and finishing welding only, not suitable for joint welding
- malleable cast iron
- abrasion resisting cast irons (not weldable)
- austenitic cast irons
- ausferritic spheroidal graphite cast irons
In particular a number of practical examples, taken from the literature or from company information, are included, with process information and photographs of the welded components such as exhaust manifolds and cylinder blocks.
The majority of examples are on ductile cast irons for which more information and experience is available, however there are also referenced examples for the other weldable types of cast irons.
The final section includes recommendations for the welding of the different type of cast irons (these are based on Table B.1 of BS EN 1011-8:202004(3).
Details of welding processes, weld preparation, pre and post weld heat treatment and filler materials are included and there is a full bibliography.
1. Cast Irons: Welding. PD ISO/TR 10809-2: 2011
2. Cast irons: Materials and properties for design. PD ISO/TR 10809-1:2009.
3. EN 1011-8:2004, Welding - recommendations for welding of metallic materials - part 8: welding of cast irons.
Other relevant standards and reports*
PD ISO/TR 10809-1:2009 - Cast irons - Materials and properties for design
ISO 1071, Welding consumables - covered electrodes, wires, rods and tubular cored electrodes for fusion welding of cast iron - classification
BS EN 287-6 2010, Qualification test of welders - fusion welding - cast iron
ISO/TR 15608, Welding - guidelines for metallic materials grouping system
ISO 15614-3: 2003 Specification and qualification of welding procedures for metallic materials - welding
procedure test - part 3: fusion welding of non-alloyed and low alloyed cast irons
BS EN ISO 11970:2007 Specification and approval of welding procedures for production welding of steel castings
* This list is not exhaustive but for more information on standards or Drafts for Public Comment (which, if approved, become full standards) relevant to castings, contact Pam Murrell at ICME on Tel: +44 (0) 121 601 6979 or email: [email protected]
To find out more about the range of cast irons and their metallurgy, contact ICME for details of forthcoming training courses, www.icme.org.uk/training.asp
To subscribe and receive all the printed issues of Foundry Trade Journal contact Yvonne Marriott Tel: 44 (0)121 601 6979 or email [email protected]
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