DUCTILE IRON DATA FOR DESIGN ENGINEERS

 

 

Home Page

SECTION VIII.  
WELDING, BRAZING, DIFFUSION BONDING, ADHESIVE BONDING

Introduction
Although the complex shapes produced by the casting process have enabled castings to replace many fabricated components, there are many applications in which, for economic or engineering reasons, castings themselves become part of a fabrication and are joined to other castings or other materials. Although often more cost-effective than steel castings and forgings, Ductile Irons have not been used in some applications requiring joining by welding because they have been considered difficult to weld. This poor weldability of Ductile Iron is partly fact but primarily misconception. When Ductile Iron castings are repaired or joined by fusion welding their high carbon content can cause the formation of carbides in the fusion zone (FZ) and martensite in both the FZ and heat affected zone (HAZ) adjacent to the FZ. The formation of hard brittle phases in the FZ and HAZ can cause a significant deterioration in both machinability and mechanical properties.

Following an investigation into the weldability of various types of cast irons, the American Welding Society Committee on Welding Cast Irons has developed both a weldability test and a set of recommended practices for welding cast irons. The weldability test consists of the production of carefully controlled autogenous welds (an autogenous weld is one made without filler metal) on test castings preheated to various temperatures and the determination of a minimum temperature, called the "no-crack temperature" above which there is no cracking in the test weld. The committee found no correlation between the no-crack temperature and the carbon equivalent (CE) formula used to determine the weldability of steels and the following formula for CE was developed.

CECI = %C + 0.31 (%Si) + 0.33 (%P) + 0.45 (%S) +
0.028 (%Mn + %Mo + %Cr) - 0.02 (%Ni) - 0.01 (%Cu)

Figure 8.1 shows that there is a good correlation between CEC, and the no-crack temperature for Gray, Ductile and Malleable irons. The autogenous welding method used to obtain this correlation was chosen to simplify and standardize test procedures and is not considered good welding practice for cast irons. For this reason CECI should be used only to rank weldability rather than determine either absolute weldability or specific preheating conditions. Through the use of welding practices and consumables described in the Guide for Welding Castings and other references used in this Section, Ductile Iron castings have been joined successfully to other Ductile Iron castings and to steel in the fabrication of automotive and other engineering components. In addition, nonfusion joining processes such as brazing, diffusion bonding and adhesive bonding can be used to produce high quality joints between Ductile Iron and a wide variety of other materials.

Back to Top

WELDING

Welding involves the fusion of both a filler metal (welding consumable) and the base metal adjacent to the weld zone. The high carbon content of Ductile Iron can lead to the formation of carbides in the fusion zone (FZ) and martensite in both the FZ and heat affected zone (HAZ) adjacent to the FZ unless correct procedures are followed. However, with the use of appropriate materials and procedures, Ductile Iron castings can be successfully joined to other Ductile Iron castings and to steel by fusion welding.

Back to Top

ARC WELDING

Several methods have been employed successfully to arc-weld Ductile Iron to itself and other materials with acceptable properties in both the weld and base metal. The properties of shielded metal arc welded Ductile Irons were greatly improved by the introduction over 30 years ago of the high-Ni and Ni-Fe electrodes (AWS Ni-CI and ENi-Fe-CI). These electrodes produce high-nickel fusion zones that are relatively soft and machinable but have adequate tensile strength, ductility and fatigue strength. The short arc, or dip transfer MIG welding process, by virtue of its controlled, low heat input, reduced harmful structural changes in the base metal HAZ. Combining the benefits of Ni-base filler wire with the short-arc MIG process has resulted in welds with tensile properties that are equivalent to the base Ductile Iron (Table 8. 1) and fatigue strengths that are 65% and 75% respectively of the fatigue limits of unwelded pearlitic and ferritic Ductile Irons (Figures 8.2 and 8.3). Although suffering from the disadvantages of high consumable costs, low deposit rate (1.8-3.2 kg/h (4-7 lb/h)) and a tendency toward lack-of-fusion defects, short-arc MIG welding has been used successfully for the joining of Ductile Iron castings for commercial applications. Recent work at BCIRA has shown that short-arc MIG welds made with high Ni filler wire have Charpy fracture energies that are superior to those of MIG-welded joints made with Ni-Fe and Ni-Fe-Mn wires and flux-core arc welded joints produced with Ni-Fe wire.

Back to Top

Table 8.1  Average transverse tensile properties of short-arc mig-welds between 25mm (1 inch) thick plates.

Matrix
structure
Filler metal 0.2% yield strength,
N/mm2       (ksi)
Tensile strength,
N/mm2     (ksi)
Elongation, %
in 50 mm (2 in.)
Ferritic Unwelded
Nickel 61
Monel 60
Nilo 55
232-309 (34-45)
(44)
(44)
(44)
386-541
422
400
412
(56-78)
(61)
(58)
(60)
15-25
11.5
8.7
12.7
Pearlitic Unwelded
Nickel 61
Monel 60
Nilo 55
386-463
358
346
339
(56-67)
(52)
(50)
(49)
618-772
550
495
425
(90-112)
(80)
(72)
(62)
1 - 3
3.5
2.5
5.7

Back to Top

Flux Cored Arc Welding
Flux cored arc welding (FCAW), utilizing a flux cored wire developed specially for the welding of cast irons, has improved upon the metallurgical advantages provided by the Ni-rich consumables and offers the additional advantage of much higher metal deposit rates (6-9 kg/h (13-20 lb/h)). The key to the success of the FCAW process is the consumable, marketed under the trade name "Ni-Rod FC55", which consists of a nickel-iron tubular wire filled with carbon, slagging ingredients, and deoxidizers. In addition to the advantages offered by the high nickel content, Ni-Rod FC55 provides the additional benefits of a high carbon content, which produce graphite precipitates during the solidification of the weld metal. It has been claimed that the expansion resulting from the formation of graphite counteracts weld-metal shrinkage, reducing stress-induced cracking of the weld. The high productivity of the FCAW method, and the good mechanical properties of welded joints (Table 8.2) have resulted in its use in the production of critical, high volume automotive components such as drive shafts, "half-shafts" and wheel spindles on off-road vehicles. This ability to economically produce high quality welds has given foundries the added freedom to employ cast-weld techniques for the production of complex components.

Recently a nickel-iron-manganese alloy, "Ni-Rod 44", with a nominal composition of 44 % Fe, 44 % Ni and 11 % Mn was developed to further reduce the risk of cracking in the HAZ. Available as both filler wire and manual electrodes, Ni-Rod 44 has been evaluated using various welding procedures on both ferritic and pearlitic Ductile Irons. Table 8.3 shows that Ni-Rod 44 welded joints have good strengths but lower ductility, compared to MIG-welded joints produced with a high-nickel consumable.

Back to Top

Table 8.2   Mechanical properties of joints welded with flux-cored wire Ni-Rod FC55:  base material ASTM grade 60/45/10 Ductile Iron.

Specimen Shielding 0.2% offset
yield strength,
N/mm2     (ksi)
Tensile
strength,
N/mm2     (ksi)
Elongation
%
Reduction
of area %
Hardness,
HRB
All-weld metal None 310 (45) 476 (69) 15.5 14.5 81
All-weld metal CO2 314 (45) 496 (72) 21.0 18.8 80
All-weld metal Sub-arc
flux
338 (49) 510 (74) 18.5 20.6 86
Transverse None 300 (44) 455 (66) -- -- --
Transverse CO2 303 (44) 455 (66) -- -- --
Transverse Sub-arc
flux
310 (45) 441 (64) -- -- --
All-weld-metal* CO2 303 (44) 468 (68) 15.0 16.2 80
Transverse* CO2 300 (44) 467 (68) -- -- --
*Pulsing-arc power source.

Back to Top

Table 8.3  Properties of welds made between 19mm (0.7 in.) thick Ductile iron plates using
a 44% FE-44% Ni-11% Mn filler wire.

Matrix
structure
Welding*
process
Protective gas Yield strength
N/mm2      (ksi)
Tensile strength,
N/mm2       (ksi)
Elongation
%
Reduction
in area %
Ferritic Unwelded -- 323 (47) 481 (70) 13 14.7
  MIG Argon 366 (53) 445 (65) 2.3 10.7
    Argon 2% O2 387 (56) 482 (70) 5.0 20.5
    Stargont 385 (56) 455 (66) 2.3 5.6
    75% Ar - 25% CO2 397 (58) 493 (72) 3.0 8.5
    CO2 390 (56) 499 (72) 2.7 14.0
  SAW -- 341 (49) 498 (72) 6.0 8.2
  TIG Argon 392 (57) 507 (74) 5.0 19.5
  MMA -- 365 (53) 490 (71) 8.3 15.3
Pearlitic Unwelded -- 413 (60) 693 (100) 6 4.7
  TIG Argon 503 (73) 629 (91) 1.5 3.2
  MMA -- 447 (65) 580 (84) 3.0 1.6

*MIG:  metal inert gas
*SAW:  submerged arc welding
tTrademark of Linde (Union Carbide).

TIG:  tungsten inert gas
MMA:  manual metal arc

Back to Top

Gas Welding
Gas welding can be used to join Ductile Iron components by the creation of either fusion or diffusion bonds. Gas fusion welding is a well established welding method for joining Ductile Iron. The process simply involves fusion of the base metal and filler rod by heat generated from an oxyacetylene flame. The weld pool is constantly being fluxed. When Ductile Iron filler rods are used and fluxes of suitable composition (usually incorporating cerium and/or other rare earth elements) are used the weld deposit solidifies like a Ductile Iron, with the formation of graphite spheroids. Successful gas fusion welding depends upon controlled preheating of the workpiece, maintenance of a controlled and well fluxed weld pool and the use of suitable consumables. The major disadvantages of this process are low productivity, dependence on operator skill and the distortion of complex castings by excessive heat input. However, when correct procedures and materials are employed, gas fusion welding can produce joints with strength and ductility properties comparable to the base metal.

Back to Top

Powder Welding
Powder welding is a non-fusion form of gas welding in which a modified oxyacetylene torch serves as both a powder supply and heat source. The melting point of the deposited powder is below that of the base iron and when the base iron surface reaches a certain temperature, the deposited powder coating melts and "wets" the casting surface. Subsequently the weld is built up as the preheated powdered alloy continuously melts as it impinges on the wetted surface. Powder welding does not fuse the base iron and the success of the weld is determined by the development of a diffusion bond. Powder welding has several limitations. It is slow, expensive and is restricted to horizontal welding. Although the casting is not heated to its melting point, sufficient heat may be applied to cause distortion in complex castings. Powder welding is used for defect repair, cladding and joining high alloy irons. Work at BCIRA has shown some promise for the joining of ferritic and pearlitic Ductile Irons.

Back to Top

Significant Welding Variables
Selection of the correct welding procedure and consumable is a necessary but not sufficient condition for the production of high quality welds in Ductile Iron. Other critical variables are:

  • type and composition of the base Ductile Iron,
  • design and preparation of the welded joint, and
  • control of the thermal history of the component before, during and after welding.

Iron Type and Composition
Although the cast iron weldability test indicates that the "no-crack temperature" is related to composition but not microstructure (Figure 8. 1), ferritic Ductile Irons are generally considered to have the highest weldability of all grades of Ductile Iron. Composition influences weldability primarily through CECI - the higher CECI is, the more susceptible the casting is to cracking. Composition also affects weldability through its influence on the hardenability of the HAZ. Manganese and chromium strongly increase hardenability, which reduces weldability through the increased tendency to form martensite in the HAZ. Although silicon increases hardenability slightly, this effect on weldability is offset by the strong graphitizing effect of silicon, which improves weldability by reducing carbide formation.

Back to Top

Joint Design and Preparation
The design of a welded joint is dependent upon factors such as metal thickness, casting geometry, welding process and service requirements. Whenever possible, the design should ensure that the components being joined, rather than the weld, carry most of the load. With a welded assembly the designer can often position the weld in an area of low stress. Figure 8.4 provides examples of joint designs which have been improved to reduce joint stress and increase weld penetration, while Figure 8.5 illustrates recommended joint designs for both welding and brazing. To ensure sound, gas-free welds, the casting skin adjacent to the joint should be removed and the joint surfaces should be freshly ground or machined and any scale, rust, dirt, grease and oil removed.

Back to Top

Thermal Treatments
When practical, the casting should be preheated in order to prevent thermal cracking, reduce hardness in the HAZ and reduce residual stresses and distortion. It is preferable that the entire casting be preheated but when casting size or the lack of facilities makes this impractical, castings can be preheated with burners or an oxyacetylene torch. When local preheating methods are employed, extreme care is required to avoid rapid, non-uniform heating to avoid cracking and distortion in complex castings. Ferritic Ductile Irons require only a mild preheating in the range 300-400oF (150-200oC). Pearlitic Ductile Iron requires higher preheating temperatures, 600-650oF (315-340oC). Low heat input welding methods such as short-arc MIG minimize the harmful effects of the HAZ. Post-weld thermal treatments such as slow cooling and postheating may be required to reduce residual stresses. Depending upon service requirements, the welded assembly may be subjected to annealing or normalizing heat treatments to dissolve carbides and produce the desired mechanical properties.

Back to Top

BRAZING
The formation of less than optimum microstructures in both the FZ and HAZ during the fusion welding of Ductile irons makes non-fusion joining techniques attractive alternatives. Brazing is "the joining metals by the fusion of non-ferrous alloys that have melting points above 800oF (425oC) but lower than those of the metals being joined". During the brazing process the melted filler metal flows by capillary action into a narrow gap between the components and solidifies to form a bond. Brazing is related to soldering, braze-welding, and powder-welding, but is distinguished from these processes either by the type and melting range of the filler metal or by the design of the joint (Table 8.4).

Back to Top

Table 8.4  Non fusion joining processes.

 

Characteristics

Process

 

Soldering

Brazing  Bronze-welding
(or braze welding)
 

Powder-
welding

Low-
temperature
High-
temperature
Melting-point or
melting-range of filler-
metal

Typical filler-
metal-alloys

Joint type
<450oC
(840oF)


Pb-based
Zn-based

Capillary
400-1000oC
(750-1830oF)


Ag-based
Cu-based

Capillary
=1000oC
(=1800oF)


Ni-based
Au-based
Pd-based
Capillary
Not specified



Cu-based
(usually Cu-Zn)

Large gap or
external fillet
Not specified



Ni-based
Cu-based

Large gap or
external fillet

Back to Top

Joint Design and Preparation
Unlike welded joints, the joint-gap for brazing (Figure 8.5) is narrow and of controlled thickness to maximize joint strength, induce penetration of the brazing alloy by capillary flow, and reduce the amount of brazing alloy consumed. The joints should preferably be designed to operate in compression or shear. Although brazed joints can have excellent mechanical properties under pure tensile loading, any bending moment will severely reduce the mechanical properties. Ductile Iron should be prepared for brazing by removal of the casting skin, roughening of the surface with an abrasive, degraphitization of the joint surfaces with an oxidizing oxy-acetylene flame or a salt bath and degreasing and cleaning with a suitable solvent.

Back to Top

Heating
The choice of a heating method for brazing depends on the component size, joint design, brazing alloy, and production rate. Brazing Torches can be hand operated, which is flexible but requires considerable operator skill, or used as fixed heat sources in a mechanized brazing line. Induction brazing is a rapid and reproducible heating method generally used on long production runs. Batch or continuous furnaces are frequently used when the entire component is heated to the brazing temperature. Brazing furnaces may have inert or reducing atmospheres or a vacuum to prevent oxide formation on both the workpiece and brazing alloy, or an air atmosphere may be used, in which case a brazing flux is required.

Back to Top

Diffusion Bonding
Diffusion bonding, in which both similar and dissimilar metals can be joined by solid state diffusion processes, can be used to overcome the microstructural problems related to fusion welding while providing a joint that is significantly stronger than that produced by other non-fusion processes. The use of a Ni foil varying in thickness from 10-100 um (0.0004-0.004 in.), with bonding temperatures and times of 820oC (1510oF) and 30 minutes has resulted in bonds between Ductile Iron and carbon steels with exceptional mechanical properties. These bonds have impact properties and endurance ratios equal to the base Ductile Iron (Figures 8.6 and 8.7) and a joint efficiency (ratio of the tensile strength of the joint to that of the base metal) which decreases from 98% to 92% as the strength of the Ductile Iron increases from 400 MPa (58 ksi) to 700 MPa (100 ksi).

Back to Top

Adhesive Bonding
Adhesive bonding is being used increasingly for the joining of engineering materials, especially sheet metals. In addition to the elimination of structural changes in the base metal, the absence of heat input in adhesive bonding also eliminates the problem of distortion and permits the bonding of Ductile Iron to a wide variety of metallic and non-metallic materials, regardless of their melting points or physico-chemical properties. The most common adhesives used in structural metal-to-metal bonds are: anaerobics, toughened acrylics and epoxy resins. Adhesive bond strengths are significantly lower than the strength of ferritic Ductile Iron and as a result, careful consideration must be given to joint design in applications in which strength is a requirement. Figure 8.8 illustrates typical examples of adhesive joints. Enhanced joint performance can be obtained through specialized joint designs which convert tensile and shear stresses into compressive stresses. Other limitations to the use of adhesive joints are their limited operational temperature range and a general lack of data on the performance of adhesive bonding in long term applications involving different loading and environmental conditions.

Back to Top

REFERENCES
Guide for Welding iron Castings, American Welding Society, 1989, 550 N. W. Lejeune Road, P.O. Box 351040, Miami Florida 33135. "Arc Welding of Cast Irons." Metals Handbook, Volume 6, 9th Edition, American Society for Metals, Metals Park, Ohio 44073

S. I. Karsay, Ductile Iron II, Quebec Iron and Titanium Corporation, 1972.

R. D. Forest, "Welding Ductile Iron Castings," Casting Engineering and Foundry World, Spring, 1983.

D. G. Howden, "Designs for Effective Welding," Machine Design, May 25, 1989.

I. S. Matharu and K. Selby, "The Charpy v-notch and tensile properties of a ferritic ductile iron welded using MIG and FCAW methods." BCIRA Research and cast metals practice, July 1990, pp 195-201.

J. L. Osman, N. Stephenson, and D. R. Thorneycroft, "Developments in the Welding of S. G. Iron," The Ductile Iron Bulletin, November, 1963

H. L. Morgan, "Joining of iron Castings," Proceedings of the BCIRA Conference on engineering properties and performance of modern iron castings, Loughborough, England, April, 1970.

R. A. Harding, "Progress in joining iron castings," British Foundrymen, November, 1987 pp 444-455.

J. L. Osman and N. Stephenson, "Further aspects of the welding of SG iron," Institute of Welding and British Welding Research Association, Second Commonwealth Welding Conference, London, England, 1966.

R. C. Voigt and C. R. Loper, "Welding Metallurgy of Gray and Ductile Cast Irons," Transactions, American Foundrymen's Society, Vol. 94, 1986.

R. A. Bishel, "Flux -cored electrode for cast iron welding," Welding journal, June, 1973.

R. A. Bishel, and H. R. Conaway, "Flux-cored Arc Welding for High- quality joints in Ductile Iron," Transactions, American Foundrymen's Society, Vol. 84, 1976.

S. D. Kiser, "Production Welding of Cast Irons.", Transactions, American Foundrymen's Society, Vol 85, 1977, pp 37-42.

M. A. Davila, D. L. Olson and T. A. Freese, "Submerged Arc Welding of Ductile Iron.", Transactions, American Foundrymen's Society, Vol 85, 1977, pp 79-86.

T. J. Kelly, R. A. Bishel, and R. K. Wilson, "Welding of Ductile Iron with Ni-Fe-Mn filler metal," Welding journal, March, 1985.

R. Medana, E. Natale, M. S. Remondino, "High Strength Welding of Ductile Iron Castings." Transactions, American Foundrymen's Society, Vol 86, 1978, pp 611-616.

The Iron Castings Handbook, Iron Castings Society, Inc., 1981.

E. N. Gregory and S. B. Jones, "Welding Cast Irons," Proceedings, International Conference on Welding, the Welding Institute, Cambridge, England, 1977.

K. Nishio, M. Katoh and S. Mukae, "Fatigue Strengths of Diffusion Bonds of Spheroidal Graphite Cast Irons." Transactions, Japan Welding Society, Vol 19, April 1988, pp 17-2 7.

S. Mukae, K. Nishio, M. Katoh and N. Nakamura, "Impact Characteristics of Diffusion Bonds of Ferritic Spheroidal Graphite Cast Iron." Transactions, Japan Welding Society, Vol 21, April 1990, pp 41-51.

Back to Top