Surface Thermal Microcracking of Cemented Tungsten Carbide Tiles During Welding and Hardfacing
======================================================================
Why a fine surface indication after hardfacing is not, by itself, a failed tile
Cemented tungsten carbide tiles are placed on downhole tools because they hold hardness and abrasion resistance under conditions that wear steel quickly. A common assembly uses small tiles, often on the order of 13 mm × 5 mm × 5 mm, positioned on a steel body, tack-secured, and then surrounded by tungsten carbide hardfacing.
That installation step is thermally different from sintering. Heat is local. The tile, the hardfacing deposit, and the much larger steel body do not heat or cool at the same rate. Fine surface cracks, often called thermal microcracking or thermal checking, can appear in the carbide afterward.
Published research shows this response is characteristic of cemented tungsten carbide exposed to a steep thermal gradient. A surface indication of this type should not automatically be read as a structural fracture, or as evidence that the sintered tile was defective. Crack depth, location, thermal history, grade, and the loads the part will see still have to be judged before anyone calls the component unsuitable.
What the installation actually does to the tile
During tack attachment and hardfacing, heat concentrates in a small area. Nearby carbide and the steel substrate stay cooler. Cooling then reverses the gradient. Cemented carbide does not respond to that cycle the way a homogeneous steel part does.
WC-Co is a composite. The tungsten carbide grains supply hardness and wear resistance. The cobalt binder supplies toughness and holds the grains. Those two phases expand and contract differently. Lagerquist, working at the Coromant Research Centre of Sandvik, tied thermal-fatigue crack nucleation and propagation in WC-Co to stresses from that mismatch, and found that grain size, cobalt content, and binder-phase dimensions affect how cracks propagate.
The practical point is metallurgical, not cosmetic. Thermal microcracking is a recognized response of the composite to thermal cycling. It is not, by itself, proof of bad carbide.
How a surface microcrack forms
A rapid temperature change leaves the surface and the interior at different temperatures, so expansion and contraction are not uniform. Ishihara, Shibata, Goshima, and McEvily studied this in cemented carbides and cermets and described tensile stress at the surface from differential thermal contraction. In susceptible material, those stresses open microcracks. They also separated that step from unstable fracture: subcritical microcracks can start before the component breaks.
That distinction matters on the inspection bench. A line you can see, or find under a microscope, does not prove the crack has crossed the tile or reduced the load-bearing section. It may be thermal damage confined to the surface.
How shallow published microcracks actually are
A 2018 study in the International Journal of Refractory Metals and Hard Materials examined thermal-shock damage in WC cemented carbides with cobalt and Co-Ni-Cr binders. Damage showed up as microcracks near the surface, preferentially at WC-metal interfaces, at about 10 to 20 micrometers from the surface.
Scale that against a production tile. One millimeter is 1,000 micrometers. A 5 mm tile is 5,000 micrometers thick.
Indication | Published depth | Share of a 5 mm tile |
Shallow thermal-shock microcrack | about 10 μm | about 0.2% |
Upper end of reported range | about 20 μm | about 0.4% |
Nominal tile thickness | 5 mm (5,000 μm) | 100% |
Those depths come from particular grades and controlled thermal-shock tests. They are not the crack depth in any given production tile, and a welding or hardfacing cycle is not the same experiment. They do establish the principle: thermal shock in cemented tungsten carbide can crack a very shallow layer and leave the rest of the section intact. Seeing a surface microcrack does not, by itself, establish a through-thickness fracture.
Microcrack versus structural crack
Treat these as different conditions, even when both show up as a line on the surface.
A superficial thermal microcrack is a local discontinuity from thermal stress, confined to a shallow region of the carbide.
A structural crack extends far enough to cut the effective section, propagate under service load, cause fragmentation, or otherwise compromise the job the tile was installed to do.
Tarragó and coauthors evaluated residual strength in WC-Co after abrupt temperature changes and framed thermal-shock resistance as resistance to both initiation and propagation. Their work is also a warning. Severe or repeated thermal shock can degrade strength. It is not defensible to say every thermally induced crack is harmless.
The claim that holds is narrower. Superficial thermal microcracking, on its own, is not sufficient evidence that the tile has suffered structurally significant damage.
Why the indication often appears after hardfacing
The tile, the hardfacing, and the steel body have different thermal properties and different mass. Heat goes in and comes out at different rates, so the gradient across a small tile can be steep. Hot regions want to expand while cooler regions hold them. On cooling, surface and interior contract at different rates. At the grain scale, WC and binder add another mismatch.
Surface indications can therefore start during attachment or hardfacing rather than in the sinter. If a tile was visually acceptable before installation and lines appear only after localized heating, thermal microcracking belongs on the list of causes. It should be considered before the tile is rejected as a material defect.
What it means for wear
On many downhole tools the tile exists to resist abrasion, erosion, and contact with formation or casing. A shallow surface microcrack does not remove the hardness of the surrounding WC grains, and most of a 5 mm tile can remain unaffected.
That is especially true when the tile is supported by the steel body and locked in by carbide hardfacing. A stable superficial indication in a mainly compressive, abrasive environment is a different engineering problem from a deep crack under tension or bending, where the load can drive the crack. Disposition should rest on evidence that the cracking is shallow, not on appearance alone.
What to check before you disposition the part
When indications show up after welding or hardfacing, look at:
Where the indication sits, and how deep it appears to go.
Whether it was present before the thermal process.
Whether it penetrates substantially into the tile.
Whether the tile is chipped, separated, or loose in the hardfacing.
Where the customer’s quality requirements call for it, section a representative tile perpendicular to the indication, polish, and examine it. That measurement separates a shallow thermal microcrack from a deeper fracture, and it can be set next to the published record of near-surface thermal-shock cracking in WC cemented carbides.
What the research supports
Thermal shock and thermal fatigue are recognized mechanisms that produce microcracking in cemented tungsten carbide. The cracks come from thermally generated stress, including the mismatch between WC grains and the metallic binder. Experimental work has documented thermal-shock microcracks about 10 to 20 μm from the surface, preferentially at WC-metal interfaces. Initiation of those cracks is distinct from unstable fracture or through-thickness cracking.
For a 5 mm tile, 10 to 20 μm is about 0.2 to 0.4 percent of nominal thickness. That comparison shows how superficial this class of damage can be. It does not measure the crack in a specific production part. Where the distinction matters, metallography or another validated method should establish whether the indication stays at the surface or runs into the tile.
A surface indication alone is not enough to conclude that a cemented tungsten carbide tile has suffered a structurally significant fracture, or that its wear function is lost. Where examination shows the indications are shallow, stable, confined to the surface, and free of meaningful propagation, fragmentation, or loss of retention, the condition may reasonably be evaluated as superficial thermal microcracking rather than a bulk fracture.
Final acceptance still belongs to the engineering specification, the inspection criteria, and the customer requirement.
References
Lagerquist, M. “A Study of the Thermal Fatigue Crack Propagation in WC-Co Cemented Carbide.” Powder Metallurgy, Vol. 18, Issue 35, 1975, pp. 71–88. DOI: 10.1179/pom.1975.18.35.006.
Ishihara, S., Shibata, H., Goshima, T., and McEvily, A.J. “Thermal Shock Induced Microcracking of Cermets and Cemented Carbides.” Scripta Materialia, Vol. 52, Issue 7, 2005, pp. 559–563. DOI: 10.1016/j.scriptamat.2004.12.002.
Tarragó, J.M., Serra, I., Al-Dawery, I., and Llanes, L. “Residual Strength of WC-Co Cemented Carbides After Being Subjected to Abrupt Temperature Changes.” Proceedings of the 14th European Inter-Regional Conference on Ceramics, Stuttgart, 2014, pp. 47–52.
“Comparison of the Thermal Shock Resistance of WC Based Cemented Carbides with Co and Co-Ni-Cr Based Binders.” International Journal of Refractory Metals and Hard Materials, Vol. 72, 2018, pp. 89–96. DOI: 10.1016/j.ijrmhm.2017.12.021.
Emanuelli, L., Pellizzari, M., Molinari, A., Castellani, F., and Zinutti, E. “Thermal Fatigue Behaviour of WC-20Co and WC-30(CoNiCrFe) Cemented Carbide.” International Journal of Refractory Metals and Hard Materials, Vol. 60, 2016, pp. 118–124. DOI: 10.1016/j.ijrmhm.2016.06.014.
Technical note
This article describes established metallurgical mechanisms and published experimental observations. It does not establish the depth, severity, or acceptability of cracking in any individual component without examination. Welding procedure, hardfacing process, carbide grade, geometry, heating and cooling rates, substrate configuration, and service loading can all influence thermal damage, and each has to be weighed when a specific part is dispositioned.
Southern Carbide Company supplies custom tungsten carbide tiles and wear components for downhole and industrial service. If you are reviewing a hardfaced assembly or specifying tile grade for a welded installation, send the drawing and the inspection notes.
Comments