"Troubleshooting Hard-Surfacing Problems"
Part Five, CRACKING
Cracking can occur at two different times during the thermal spray operation; during the spraying and after fusing. In either case, thick deposits are more apt to crack. In many applications, cracked overlays, while unsightly, are not actually detrimental to good service life.
Cracking during spraying is usually a result of insufficient preheating of the base metal, and often occurs on austenitic steels (300 series) because of their high coefficient of expansion. The remedies are to preheat to a higher temperature, to use a thinner coating or to traverse the spray gun at a faster rate.
Cracking after fusing is usually associated with cooling the part too fast. Slow cooling is advisable on non-hardening steels such as AISI 1015 because the coating is cooling and contracting faster than the base metal. Since most of the hardsurfacing alloys have practically no ductility, they crack rather than yield to accommodate the tensile stresses involved. The remedy is to control the cooling rate, based upon a correct identification of the base metal and knowledge of its properties.
Hardenable steels are generally classified as water-hardening, (carbon steels), oil-hardening (low alloy steels) and air hardening (high alloy steels). These classifications are based on cooling rates, which determine the degree of hardening. For our purposes, however, these classifications cannot be strictly applied, since section size determines the cooling rate. For example, 4140 is classified as an oil-hardening steel, yet in a ¼ inch section it I air hardening, a two-inch section is oil hardening, and a six-inch section is water hardening. By substituting different section sizes this example is true for all of the hardenable carbon and low alloy steels. This is why overlays on thin sections are more prone to cracking than coatings on thick sections, even when the steel is the same type.
Following is a general description of a number of grades of steel, along with a review or their properties that contribute to cracking, and suggestions for control through cooling and isothermal annealing.
17-4 or 17-7 PH Cracking of the deposit will occur on these steels despite the use of any known method of slow cooling or annealing.
AISI 424 and 431 This group of steel contain nickel which retards the transformation to such an extent that it is not practical to attempt isothermal annealing. With the exception of 17-7 PH, both of the above groups of steels can be hard-surfaced successfully if cracking can be tolerated. 400 series stainless Certain of the steels in this family: 405, 430, or 446, are ferretic and thus non-hardenable and less prone to cracking. Like all 400 series, however, they have a low coeffiecient of expansion and therefore must be slow cooled to assure they won’t crack. The remainder of the 400 series: i.e. 403, 410, 416, 420, 440A, 440B and 440C are all air hardening. The fastest, most practical way to prevent cracked overlays on these steels is to isothermal anneal them. This is accomplished by transferring the part, immediately after fusing, to a 1300°F (704°C) furnace, and holding at this temperature for a minimum of three hours. Do not allow the part temperature to fall below 600°F (316°C) during the transfer. The nickel content of the steel being annealed should be taken into consideration in determining the holding time. For example, a 410 stainless with a residual nickel content of 0.60% must be held at 1300°F (704°C) for 8 hours to completely transform to the soft ferrite and carbide microstructure.
With any of the hardenable steels, cracked deposits often occur on cylindrical objects, such as shafts, where only a portion of the length is hard-surfaced, and the part is placed in an insulating media immediately after fusing. In such cases, the unfaced area acts as a heat sink, drawing in a quenching effect, and cracking of the overlay. The solution here is to heat the uncoated area to the same temperature used for fusing before placing the part into an insulating media.
Insulating media: When parts are placed in an insulating media to control the cooling rate and prevent cracking, it is essential that hardenable steels be so placed before their temperature drops below the critical upper limits, 1400 to 1500°F (760 to 816°C), at which transformation will begin. Acceptable insulating media are Vermiculite (expanded mica) or Sil-O-Cel. Sand and lime are not recommended. Workpieces should be completely surrounded by at least three inches (76.2 mm) of media.
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