Welding of Gray and Ductile Iron
The welding of gray and ductile cast iron is often considered unfeasible. The chemical makeup of steel compared to cast iron is not hugely different in some cases, other than the higher carbon content in the cast iron. Therefore, it would be reasonable to think that if you can weld steel then you can weld cast iron. Unlike the welding of steel, cast iron is not going to have exactly the same structure or properties as steel. The American Welding Society (AWS) has several standards devoted to the welding of iron castings. The main standard is ANSI/AWS D11.2-89 (R2006), Guide for Welding Iron Castings. The standard was originally written and approved by ANSI in July 1988 and documents the metallurgy and weldability of the various cast iron alloys, filler metals and welding processes. The cast microstructure must be considered when choosing an appropriate welding process.
An example of a weld repair that has been performed for many years is in automotive stamping dies. The stamping dies used to fabricate the entire body side panels, hood, trunk, fenders, and doors are typically made of gray, or most often ductile cast iron. Low volume, specialty shaped hoods might be made from gray iron. However, all high-volume stamping dies are made of ductile iron. These stamping dies are often welded to change the shape of the component without having to purchase new dies. A new set of stamping dies could potentially cost hundreds of thousands of dollars and take eight to 12 weeks or more to produce. The welding practices and procedures have been thoroughly studied and documented and welders are certified to perform the welding task. Therefore, it is much more economical to make small modifications this way. Stamping plants have used weld repairs when a part or tool gets accidentally left behind in the dies (i.e., maintenance worker’s wrench). The out-of-specification wrench-shaped die is detrimental to the quality of the stamped fender. Dies are then removed, welded in the depressed area from the wrench and then machined back to specification. A similar welding method is utilized in cosmetic repairs to cast surfaces.
An example of structural welding of cast iron is in the miles of welded ductile iron pipe under ground transporting liquids with water-tight welds all across the country. There are well defined procedures for welding pipe and a qualification process for the welder.
Welding Qualification Procedures
The welding standards include the importance of generating WPS (Weld Procedure Specifications), PQR (Process Qualification Records) and WQT (Welder Qualification Tests). The purpose of the PQR is to document settings, materials, and methods to develop the WPS for a known material and application. A welder qualification test utilizes the Weld Procedure Specification for a welder to perform a weld to the specification. The welded product is then evaluated mechanically and microstructurally. If the welder passes the test to the WPS, they would then be certified to weld product that requires that specific WPS. The welder would have to recertify on an established time basis. This is covered in another AWS Standard, AWS B2.1. These welding standards are available on the AWS website at: www.aws.org.
Welds have several classifications. A Class 1 weld is a structural weld. The weld will see high stress. The failure of the weld will result in potential damage to equipment or personnel. A Class 2 weld is a weld that will not be under high stress. A failure of a Class 2 weld will result in no hazard or harm but affect equipment operation. A Class 3 weld is more of a cosmetic weld. This weld, if it fails, will not create hazard or harm, and it will not affect equipment operation. A great place to start would be a review of the American Welding Society standard ANSI/AWS D11.2-89 (R2006), Guide for Welding Iron Castings.
A few other cast iron alloys may be encountered, especially in the agricultural industry. White iron is often utilized for components that will experience abrasive conditions (i.e., chisel plow point). Other white iron applications are in the mining industry, digging through miles of earth. Attempting to weld white iron typically results in cracking.
Lower quantities of malleable iron and compacted graphite irons are cast for various applications. Due to the similar matrix structures and free graphite in these irons, they should weld similar to gray and ductile iron parts.
As with most welding applications, there are several methods that can be utilized or selected based on the application. Probably the most used are welding rods. The welding standard, AWS A5.15:1990(R2016)1 Specification for Welding Electrodes and Rods for Cast Iron covers welding rods, shielded metal arc welding electrodes, gas metal arc and flux-cored arc welding electrodes. The main concentration of this paper with regards to welding methods will be in the chemical and mechanical properties of the weld metal available.
The American Welding Society standard provides a guide for weld metal manufacturing with regard to chemistry. The standard for welding cast irons is: AWS A5.15:1990(R2016) Specification for Welding Electrodes and Rods for Cast Iron. The American Welding Society also has standard AWS D11.2 Guide for Welding Iron Castings. This standard is a great reference guide to welding iron castings. Table 1 shows the chemical composition requirements for undiluted weld metal for shielded metal arc and flux cored arc welding.
An internet search of welding rod suppliers can produce a solid list of weld rod or wire options. Various suppliers may only share limited information, which may require an inquiry to obtain necessary material properties (i.e., mechanical properties such as tensile strength and hardness or chemical composition).
Table 2 lists welding materials by name and some of their properties. Talking with a technical representative from a weld material supplier is also helpful.
An example from Table 2 would be the use of Supercast for gray iron weld applications and Ultracast for ductile iron applications. In speaking with the sales representatives for these two products, this was the recommendation. The recommendation would be to weld a test piece with both materials and evaluate the results. In tests performed for an organization involved with automotive stamping dies, it was found that one weld material worked very well for ferritic ductile iron but did not perform well on pearlitic ductile iron.
The American Welding Standard (AWS) lists several approved weld methods.
The most common are:
Gas Metal Arc Welding (GMAW, also called MIG welding).
Shielded Metal Arc Welding (SMAW, also called stick welding).
Often, Gas Tungsten Arc Welding, (GTAW, also known as TIG welding), is assumed to be the method for welding aluminum or stainless steel. This is a very good method for welding whenever feasible. However, most industrial plants have only stick or Metal Inert Gas (MIG) welding capability. The good news is that this technology can provide robust weld joints. All these welding methods are covered in the AWS standard for welding cast irons.
The most common methods for good structural welds are SMAW, GMAW, and GTAW. In the case of machined surfaces, TIG welding might be the best option to prevent weld spatter from sticking to the machined surface.
Several considerations for welding gray and ductile cast iron should be made. The first is determining the type and grade of iron to be welded. At a minimum, knowing the chemistry and mechanical properties of the iron will be helpful in choosing the weld filler material. In some cases, iron is welded to other materials, typically steel. Some casting processes incorporate steel components in the mold to join the two materials. This is likely the easiest form of joining steel to iron. Examples include brake rotors and heavy truck drums that utilize steel mount flanges with gray iron braking surface materials. However, the steel is essentially annealed by the casting process. If the properties of the steel are not to be annealed, welding the two materials is the correct option. The next step is to find a weld material with similar mechanical properties. If there are several material options with the desired mechanical properties, weld test pieces with each of the weld materials, perform weld bend tests, and evaluate the weld under a microscope. Figure 1 is an example of a good weld interface between the ductile cast iron and the weld material.
Other than the change from the weld material to the iron, there should be no defects. A bad weld may have gas porosity or even a well-defined border between the weld material and the cast iron. This could result in failure of the weld joint. The test of the welding material on the ferritic and pearlitic ductile iron exhibited areas with both defects. The test welds were completed by one person with over 15 years of cast-iron welding experience. This supports the emphasis of testing the weld material to evaluate how the combination of weld material and base iron react.
The weld must be able to survive the same stresses the original cast part was designed for. A failed weld may not be attributed to the procedure used or the person welding but the wrong weld material selection. Another cause for a failure is when the interface of the two materials has significant porosity or poor mixing of the weld material and the cast iron.
If the casting can be shipped, weld shops with certified cast iron welders can weld the casting or weldment if joining multiple parts with rod or wire. Some weld materials have been developed that will weld “dirty” cast iron parts. The weld flux developed for these rods can eliminate the effects of the contaminants on the weld. The ideal situation is welding a cast iron part that is free of oil, grease, cutting fluids, etc. These contaminants will create gas porosity and poor bonding in most situations.
The first step when welding cast iron is to remove any casting skin. The casting skin is essentially an oxide layer that resists melting and mixing with the weld material. If feasible, it is best practice to create a “V” or “U” shaped groove for the welding (Fig. 2). This is the case whether using arc, MIG, or TIG welding. Often, a root weld is made with one weld material and method. The remainder of the weld is performed with a different weld material and possibly a different method.
Due to the high thermal conductivity and potential to generate stresses in the casting, the parts are typically preheated. It is important not to get too hot as cast iron changes characteristics above 1,400 F (760 C). Typical minimum preheat temperatures are between 200 and 750F (93-398C). In the studies conducted with the automotive stamping die customer, the target preheat was 600 to 650F (315-343C).
Another role of preheating is to burn away oil and residue in the weld area. Some of this will vary depending on the iron and possibly other metals welded to the iron as well as the weld material used. Testing on sample parts is highly recommended.
Once the part is ready to weld, make sure the welder is set in the correct range. Many weld material suppliers will supply the recommended amp setting based on the diameter of weld material.
Six essential variables that affect weld quality are:
Width of the weld pass.
Base metal chemistry.
Weld material chemistry.
Post-weld heat treatment.
A couple of standards to reference for guidance with welding cast iron parts are: Section IX of the ASME Boiler and Pressure Vessel Code and “Welding and Brazing Procedure and Performance Qualification,” military specification MIL-STD-248. The ASME Boiler and Pressure Vessel Code Section IX provides a fillable form QW-482, “Suggested Format for Welding Procedure Specifications (WPS).” This form could be used as an example to create an internal form or procedure for welding cast iron to other cast iron or cast iron to steel, etc. Form details include: Joints, Base Metals, Filler Metals, Positions, Preheat, Postweld Heat Treatment, Gas, Electrical Characteristics, and Technique.
After each weld pass, remove any slag from the weld. Veteran welders of cast iron recommend peening the weld with a ball peen hammer, a small needle scaler or blunt-end chisel. The peening process of tapping the soft weld as it cools with a ball peen hammer or similar device helps prevent cracking. This process creates a surface compressive stress to fight cracking. Repeat these steps for each weld pass required. Inspect for gas bubbles or slag trapped in the weld metal. The slag needs to be removed by die grinder, if necessary.
If there are bubbles in the weld, the part is either not free of contaminants or there is a reaction taking place during the weld that is creating the gas. There are several possible causes—but the gas porosity does create a concern. The porosity creates a location for a failure to occur. This will require going back through the checklist: were the parts clean, was the welder at the correct power output settings, was the correct weld metal being used? Once the casting is welded, slow cooling is beneficial. Quenching a weld with water or air can cause a brittle weld joint. If possible, place the component in a furnace and slow cool if this was utilized for preheat. Other options are to wrap or cover the part with an insulating blanket or sand. For the large automotive stamping dies, heat blankets are used for preheat and post-heat treatment.
Analysis of the weld under a microscope can provide insight into the quality of the weld. The examples in Figures 3-5 are all for the same chemistry ductile iron with different welding materials.
In Figure 3, note the carbides at the interface of the weld metal and the iron casting. The iron is carbon rich and a great thermal conductor. This combination with low preheat can result in carbides, which equals a brittle weld joint. A weld bend test of this joint would have a high likelihood of failure. In this case, a post heat treatment could be done to try to dissolve the carbides.
In Figure 4, a visible separation appears at the interface. The separation is likely due to contaminants on the iron or cast iron skin. Due to the lack of gas porosity in this area it is likely a cast iron skin effect—still, a small layer of carbides at the interface.
In Figure 5, a small transition zone is visible and the appearance of potentially small graphite nodules are visible in the weld material. The lack of carbides and the good mix of the base iron and weld material appears to have created a great bond between the two materials.
Welding of gray and ductile cast iron can be achieved by a variety of methods and to materials like steel. To be successful in welding these materials requires developing a best practice of matching the appropriate weld material and method to the preparation of the metal to be welded.