Welding ductile iron to steel: a new welding process will allow ductile iron castings to be welded to steel, making fabrications using both metals a reality.
This article presents the results of an attempt to improve the mechanical properties of gas metal arc welded ductile iron and steel components in order to enlarge the range of applications for such fabrications. And, as will be seen, significant improvements in properties have been made for welding ferritic and pearlitic ductile iron to mild steel.
Just In Time
The history of metal joining can be traced to the year 3000 B.C., but its development really accelerated in the late 19th century. The discovery of a process in which an electric arc is used to melt the edges of two pieces of metal and join them together allowed the rapid growth of arc welding. That process has evolved to become the prevalent welding technique for ferrous alloys.
While welding steels has been rapidly and successfully introduced in all industrial sectors, the difficulty of welding cast irons was recognized early. Because of its high carbon content, molten iron tends to solidify with a carbidic structure. This limits the ductility and toughness of the weld, which has restricted the use of cast iron parts in many fabrications that could have taken advantage of the casting process and its properties. Ductile irons with properties competing with those of steel would find many new applications if it could be welded without becoming brittle. Processes for welding ductile irons have been developed in the past, but improving the quality would enlarge the range of applications, specifically in the heavy vehicle industry.
The Road to Weldsville
To ease the road toward an improved bond between ductile iron and steel, researchers employed welding procedures that used existing equipment and could be easily introduced in industrial manufacturing processes (i.e. with an acceptable level of productivity). All laboratory and application welding tests were performed with industrial equipment and commercially available consumables (Table 1). A multi-pass approach was selected to minimize the interactions between the base materials and the heat involved during the welding process.
In order to achieve optimal results, it was necessary to use a shielding gas containing both argon and helium in combination with small amounts of some oxidizing components.
The welding procedures included pre-welding heat treatments from 300-320C (572-608F) and post-welding cooling followed by heating to 600-620C (1,112-1,148F) maintained for two hours. In order to identify the heat treatment parameters, test plates were instrumented with thermocouples to monitor the temperature profiles obtained as a function of the welding parameters.
The initial welding tests consisted of joining 0.59-in. (15-mm) thick ductile iron plates (ferritic or pearlitic) to low carbon steel; the typical chemical compositions of the materials are listed in Table 2. The test coupon assembly had a gap of 0.08-0.12 in. (2-3 mm), supported by a ceramic/copper backing, and each plate had a bevel angle of 30 degrees (Fig. 1).
[FIGURE 1 OMITTED]
The welded specimens were characterized by metallographic examination, hardness and microhardness measurements, and impact, tensile and bending tests. Impact resistance was measured in different locations of the weldments, as well as in the parent metals. Tensile properties were measured in both transverse and longitudinal directions with respect to the weld (Fig. 2).
[FIGURE 2 OMITTED]
A typical macrostructure and microhardness profile of a welded steel-to-ferritic ductile iron joint was obtained during the initial trials (Fig. 3). The microhardness profile showed a smooth transition between the steel and the weld, but a high hardness P peak occurred in the transition zone between the weld and the ductile iron. A narrow, typical fusion zone arose at the steel-weld interface. A fusion zone containing carbides was present on the ductile iron side, resulting in high hardness in the vicinity of the fusion line. The adjacent heat affected zone (HAZ) also displayed a multi-phase pearlite/ferrite/cementite structure (Fig. 4). Such a mixed structure usually shows low impact resistance (less than 5 J at room temperature) and tensile elongation (0.5%). The goal then was to minimize the extent of the HAZ and fusion zone to achieve properties approaching those of the parent ductile iron. Changes in the welding procedures allowed the reduction of the thickness of the embrittling layer to less than 0.01 in. (0.3 mm), and small graphite nodules developed in the HAZ/fusion zone (Fig. 5). Micro-hardness measurements (VHN 100 g) in the HAZ/fusion zone ranged between 200 and 250 VHN, which was significantly lower than the 600 VHN peak.
[FIGURES 3-5 OMITTED]
Testing the Process
The improved specimens were submitted to Charpy impact (Table 3) and bending tests. The range of the impact resistance values exceeded past data for similar ductile iron and steel materials.
Material used in Test F4, which tested the effect of test temperature on impact resistance, also was submitted to low temperature Charpy impact test (Table 4). The parent ductile iron's HAZ/ fusion zone and the welding material had a ductile/brittle transition temperature lower than -4F (-20C), while the steel HAZ was embrittled at low temperature.
Bending tests were carried out on welded specimens by hammering on the extremities of the ductile iron and steel sections in order to locate the maximum stress in the weld region. Cracking occurred in the parent ductile iron, but the weld remained intact (Fig. 6).
[FIGURE 6 OMITTED]
The conditions used for welding ferritic ductile iron were adapted to pearlitic irons. Some of the pearlitic ductile iron plates were low in magnesium content (Table 2), with a resulting structure consisting in a mixture of nodular and vermicular graphite particles in a pearlitic matrix. As a result, the typical tensile properties of these iron plates were those of a compacted graphite iron (UTS of 430-550 MPa and elongation of 1-2%). Nevertheless, the properties measured in the HAZ and fusion zone should have been comparable to those of welded pearlitic ductile irons, since the vermicular graphite particles dissolved faster than the nodular ones during welding because of their large specific surface area.
As for ferritic iron, its thickness was in the 0.004-0.01-in. (0.1-0.3-mm) range. It was followed by an HAZ containing fine pearlite and tempered martensite.
The typical microhardness measured in the HAZ/ fusion zone was higher than that of the ductile iron (represented by P1-P5 in Table 6) but did not reach values typical of a mostly carbidic structure. The hardness values of the HAZ/fusion zone reported in this study after annealing in the 1,112-1,202F (600-650C) range are lower than those found in the past for welds submitted to a similar heat treatment.
Will It Hold?
The properties of the welds (specimens P6, P7 and P8) were verified by impact and tensile tests at room temperature. The location and identification of specimens P1-P8 are shown in Fig. 2.
Charpy V-notch impact energy was measured in the weld metal and the ductile iron HAZ/fusion zone (Table 6). The welding rod used in past tests was Ni-61, which most probably explained the difference seen in the properties of the weld metal zones. The results obtained in the HAZ/fusion zone with the new welding procedures were clearly superior to those reported in the past.
A few tensile tests were carried out in the transverse direction of the weld. However, the results were strongly influenced by the tensile properties of the iron plates, which were those of compacted graphite iron rather than those of ductile iron (lower ductility and strength). This made them non-representative of the mechanical strength of the weld. This was confirmed by the fact that some samples failed in the cast iron section of the bar away from the weld.
Tensile tests were run on longitudinal specimens, including the following regions: the iron (No. 9), the HAZ (No. 10) and the fusion zone (No. 11). Results are presented in Table 7. As previously discussed, the poor nodularity of the graphite particles caused the samples machined in the iron to be the weakest. Those including the HAZ and fusion zone, however, exhibited tensile properties close to those of pearlitic ductile irons. In these samples, the vermicular graphite particles were dissolved rapidly, and graphite re-precipitated in a more nodular shape.
The new welding procedure reduced the thickness of the carbidic fusion zone from 0.024 in. (0.6 mm) to less than 0.01 in. (0.3 mm), and the technique was found applicable to both ferritic and pearlitic ductile irons. The toughness and tensile properties of the resulting welds significantly improved upon earlier attempts to join the metals.
For More Information
"Specifying Steel Castings Keeping Alloy Composition in Mind," J.D. Carpenter and B.R. Hanquist, Engineered Casting Solutions, Fall 2001, p. 41.
"Designing With Ductile Iron to Achieve Strength and Economy," Ductile iron Marketing Group, Engineered Casting Solutions, Winter 1999, p. 47.
M. Gagne and S. Leclerc, Rio Tinto Iron and Titanium Inc., Montreal S. Helgee, N. Stenbacka and J. Tani, Linde Gas, Stockholm, Sweden
Table 1. Welding Equipment, Parameters and Consumables Welding Equipment ESAB Auto 500 Welding Operation Mechanized Deposition Rates Root pass: 7.7 kg/hour, multi-pass: 8.6 kg/hour Shielding Gas Linde Gas MISON 2 He Filler Material Ni-rod metal 44 Wire Diameter 1.2 mm Table 2. Typical Chemical Composition of Test Materials Element Ferritic Ductile Pearlitic Ductile Mild Steel Iron Iron C 3.3-3.4% 3.6-3.7% 0.08% S 0.01% 0.008% 0.022% Si 2.4% 2.4-2.5% 0.24% P 0.017% 0.01% 0.015% Mn 0.16% 0.15% 0.5-0.6% Cr 0.027% 0.03% 0.12% Ni 0.025% 0.06% 0.08-0.13% Cu 0.058% 0.70% 0.19-0.25% Mg 0.035% 0.025-0.035% * -- * Vermicular graphite found in plates with lower magnesium content. Table 3. Impact Test Results at 20C Position Test F1 Test F2 Test F3 Test F4 Past Data Weld Metal 63/101 89-91 64-73 45 60 Fusion Zone 12-18 16-20 14-18 11-20 8, 11 HAZ 16-18 NA 14-16 -- 11, 12 Ductile Iron 12 12 12 14 -- Steel -- -- -- 65 -- Steel HAZ -- -- -- 50 -- Table 4. Effect of Test Temperature on Impact Resistance (Test F4) Region +20C -20C Ductile Iron 13.6 12.2 D.I. HAZ/Fusion Zone 13.2 11.3 Weld 41.3 30.5 Steel HAZ 50.2 19 Table 5. Microhardness of HAZ/Fusion Areas in Pearlitic Ductile Iron * (in VHN 100g) Test Number Ductile Iron HAZ/Fusion P1 314 322 P2 300 322 P3 268 297 P4 323 366 P5 277 336 Past Tests -- 420-500 * Stress relieved at 600-650C. Table 6. Results of Impact Tests for Pearlitic Ductile Iron Notch Position P6 P7 P8 * Past Results Weld 24-38 16-27 12-16 11 HAZ/Fusion 8-9 5-7 7 3-4 * High Manganese welding rod Table 7. Tensile Properties of Longitudinal Specimens Sample P6 P7 Position 9 10 11 9 10 11 Description Iron HAZ Fusion Iron HAZ Fusion YS (MPa) 344 358 363 444 450 450 UTS (MPa) 430 555 550 577 721 710 Elongation (%) 1.9 7.7 8.4 0.8 5.7 6 Sample P8 Position 9 10 11 Description Iron HAZ Fusion YS (MPa) 442 453 447 UTS (MPa) 540 583 714 Elongation (%) 1.8 1.7 5.7