Metallurgical aspects of welding Stellite 12 to Type 316L stainless steel

Applying a corrosion-resistant Stellite overlay to a stainless steel can be a daunting task, Oscar Quintero, a metallurgical and materials engineer at M&M Engineering Associates Inc, told the editors, and problematic if the welding is not done properly. Issues such as cracking along the weld line, hydrogen-induced cracking, and porosity, among others, have been reported. This article addresses issues such as lack of fusion and porosity encountered when welding Stellite 12 and Type 316L stainless steel, and offers some mitigation strategies.

Welding1Fusion/dilution issues. A microhardness profile between the base material and the Stellite overlay can give very helpful information at the dilution area. The step function on the profile graph in Fig 1 indicates that the diffusion/dilution between both materials is very poor.

Microhardness measurements at the overlay would indicate very high hardness values (usually in the Rockwell C scale) while the base material would have readings in the Rockwell B scale. The step function also indicates a lack of dilution that is usually harmful to the mechanical properties, such as tensile strength and fracture toughness, of the component.

The dilution between both materials is very important. Dilution is defined as the change in chemical composition of a welding filler metal (in this case Stellite) caused by the mixture of the base metal—or previous welds if several weld passes were made—in the weld bead.

Too little dilution could cause a stress riser at the fusion line that can potentially fail. The lack of fusion (dilution) or too little dilution can cause a hardness (or microhardness gap) creating stress risers—areas in which localized stresses are highly concentrated. If these stresses exceed the material’s strength, a crack may result and potential failure may occur. Too much dilution could reduce the wear-resistance properties associated with the hardfaced surface at the fusion area.

The example below, provided by Quintero, shows the results of microhardness testing and a map of the elemental composition of the fusion area using energy dispersive x-ray spectroscopy (EDS) between Stellite 12 and 316L stainless. Fig 1 shows the microhardness testing results of the fusion area. A steep drop in microhardness is noted between the base metal and the hardened surface in a distance of approximately 0.30 in. (7.5 mm). The reduction in hardness was close to 300 points in the Vickers scale, which converts to approximately 30.0 Rockwell C (HRC).

In addition, the dilution between the filler metal (Stellite 12) and the base metal is almost non-existent, as shown in the EDS map of the fusion line (Fig 2). This suggests that in this particular case, there was an issue in the welding process which led to this low dilution—such as an incorrect temperature causing not enough base metal and hardface layer to mix.

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Gas porosity. Porosity, or holes within the weld metal, usually occurs because of the absorption of gases and a chemical reaction. This happens when a metal susceptible to porosity dissolves large amounts of gas contaminants to the molten weld pool which are then entrapped when solidifying, Quintero said.

Contaminants can include moisture, oil, paints, and rust, as well as oxygen and nitrogen in the air. Heat from the welding arc can also decompose such contaminants into hydrogen and other gases.

Another contributor is cooling rate. The metallurgist explained that when cooling rates are fast, the level of porosity is low because the gases are suppressed and no bubbles are formed. At very slow cooling rates, porosity is also minimal because the bubbles have time to coalesce and escape from the weld pool. When the weld cooling rates are intermediate, porosity can become a problem because conditions become optimum for both formation and entrapment of the gases.

Porosity also can be associated with lack of workmanship. If the parts to be welded and the consumables are not cleaned and dried, the risk of porosity increases.

Welding3Shrinkage porosity is caused by sections of the hardface layer that solidify faster than the material around it and insufficient metal flow for a complete fill (Fig 3). This generally happens, Quintero said, when the weld area is too hot relative to the surrounding area. From another perspective, when the part is not preheated enough, the heat is quenched too fast and may also cause shrinkage porosity. Unsuitable material composition, incorrect temperatures, or a combination of these factors also can cause shrinkage porosity.

Mitigation strategies include pre- and post-weld heat treatment, he continued. Preheating the base metal immediately before welding improves the quality of the weld/overlay for these reasons:

      • Slows down the cooling rate. A slow cooling rate helps minimize porosity since the bubbles have time to coalesce and escape from the weld pool.

      • Reduces shrinkage stresses and distortion. When a drastic temperature change occurs, the material suffers shrinkage stresses and distortion. Shrinkage stresses and distortions will not go away but they can be minimized. By preheating, such stresses and distortions are minimized.

      • Promotes fusion. This raises the material’s initial temperature to ensure good weld fusion from the start. There are instances when a material with a high thermal conductivity (such as copper or aluminum) is welded onto another material. For comparison purposes, the thermal conductivity in SI units of watts per meter kelvin(W/m·K) for copper is 385; for 316L, 14-15.9; and for Stellite 12, 14.6. Since the thermal conductivity is very low for both Stellite 12 and 316L stainless steel, the deposited layer chills and cools down slowly without any fusion onto the parent material.

      • Removes moisture. Usually, it is not necessary to preheat austenitic stainless steels, unless there is condensation. If condensation is present, usually a gentle and uniform heat should remove it. Preheats higher than 212F in stainless steels can cause negative effects, such as rise to carbon pickup or metallurgical instabilities. In martensitic stainless steels, a high preheat temperature is recommended and cooling must be controlled. Ferritic stainless steels rarely are preheated.

Post-weld heat treatment (PWHT) typically is applied to increase resistance to brittle fracture and reduce residual stresses. PWHT also can reduce the hardness gradient between the base material and the weld, and enhance the material’s properties—such as ductility and tensile strength.

Usually, there is no need in austenitic stainless steels (Type 316L is one of these) for PWHT. However, PWHT after applying the overlay most likely will enhance mechanical properties—such as fracture toughness and ductility. PWHT can also reduce residual stresses and may also reduce the hardness.

By either annealing or stress relieving the component, the hardness gradient between the overlay and substrate will be reduced. A higher hardness gradient usually causes higher stress concentrations along the weld line and higher cracking potential.

Additionally, the weld and heat-affected zone (HAZ) will be prone to hydrogen-induced cracking (HIC) if any hydrogen was entrapped during the original overlaying process. Three main factors are required for HIC: stress, a sensitive microstructure, and hydrogen.

The stress source is caused by the residual stresses along the weld line. Austenitic stainless steels have a sensitive microstructure. If the fusion line becomes sensitized, it loses strength from the diffusion of hydrogen into its grain boundaries and becomes brittle (Fig 4).

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The PWHT should be performed outside the range 806F to1652F. Any PWHT performed in this temperature range will cause the chromium carbides to precipitate within the grain boundaries (sensitization) and will reduce the corrosion resistance of the alloy. In addition to reducing the hardness gradient, PWHT will stress relieve the weld line and HAZ. This also will result in an increase in fracture toughness.

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