{"id":25155,"date":"2018-10-22T15:20:34","date_gmt":"2018-10-22T21:20:34","guid":{"rendered":"https:\/\/www.boartlongyear.com\/?p=25155"},"modified":"2025-03-10T14:15:08","modified_gmt":"2025-03-10T20:15:08","slug":"understanding-and-preventing-heat-check-cracking-on-drill-rods","status":"publish","type":"post","link":"https:\/\/www.boartlongyear.com\/es\/insite\/understanding-and-preventing-heat-check-cracking-on-drill-rods\/","title":{"rendered":"Comprensi\u00f3n y prevenci\u00f3n del calor Revisar el agrietamiento de las varillas de perforaci\u00f3n"},"content":{"rendered":"
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This article originally appeared in the GeoDrilling International June 2019 issue.<\/a><\/em><\/p>\n<\/div>

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What is Heat Check Cracking?<\/span><\/h3>
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Heat check cracking is the engineering term describing the brittle cracking failure of steel, wherein a thin surface layer has become excessively hard and brittle as the result of rapid cycles of frictional heating and cooling.<\/p>\n<\/div><\/div><\/div><\/div>\n\t\t\n\t\t\n\t<\/section>

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The frictional heating is the result of \u2018rubbing\u2019 contact or \u2018drag\u2019 against a mating surface. Friction heat can build to exceed the transformation temperature of the steel (or \u2018austenite transformation start temperature\u2019, ~750degC\/1350degF), followed by rapid cooling from surrounding steel or cooling fluid, hardening, and embrittlement. When this cycle repeats frequently, the heating and cooling create rapid expansion and contraction which leads to fatigue failure, seen as perpendicular cracks that propagate from the surface.  \n<\/div><\/div>

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Figure 1 \u2013 Oil & Gas Tool Joint with heat check hardening and cracking<\/em><\/p>\n<\/div><\/div><\/div><\/div>\n\t\t\n\t\t\n\t<\/section>

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While this phenomenon is well documented in engineering texts, the problem has been prominent in the oil and gas exploration industry since the 1940\u2019s. The API (American Petroleum Institute) describes heat check cracking as, \u201cFormation of surface cracks formed by the rapid heating and cooling of the component\u201d (API \u2018RP 7G-2, Recommended Practice for Inspection and Classification of Used Drill Stem Elements\u2019, and \u2018RP 96, Deepwater Well Design and Construction\u2019.) <\/em><\/p>\n<\/div>

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A 1992 IADC\/SPE paper on heat check cracking described full-scale simulations to prove that the heating and hardening are easily achieved, but that fatigue cracking only results from the rapid heating and cooling associated with each rotation of the drill string.<\/p>\n<\/div><\/div><\/div><\/div>\n\t\t\n\t\t\n\t<\/section>

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How does Heat Check Cracking develop on Wireline Coring Rods?<\/span><\/h3>
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As shown in these pictures of wireline coring drill rods, heat check cracking can actually be easily identified, visually in the field. Heat check cracks are unique in that they follow the axis of the rod (longitudinal or \u2018axial\u2019 orientation), and are located near the female or box end shoulder, and are associated with a shiny, polished wear area. This section of the box always protrudes slightly more than any other area on a wireline drill rod and grows or \u2018bulges\u2019 under high drilling loads.<\/p>\n<\/div>

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\u2018Box bulging\u2019 is the result of a) the interference fit between the pin and the box (which is responsible for keeping the joint closed under deceleration ) and, b) the compression of the box shoulder under torsion and any radial loading from the thread-form. The positive load flank angles of traditional thread-forms, such as Q\u2122 threads, generate radial load components that can increase box bulging to the point of separation, whereas premium thread forms with reverse-angle load-flanks, such as RQ\u2122 and XQ\u2122, actually limit box bulging.<\/p>\n<\/div>

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Since heat check cracks are the result of fatigue loading, they always start from the surface and form perpendicular to the direction of the expansion and contraction which is longitudinal or axial on a drill rod and easily pass across turns of thread. Fatigue failures that result from excessive drilling loads or excessive deviation always produce cracks that start internally and form perpendicular to the axis, or circumferentially, and typically follow the thread. In other words, since there are no drilling loads that act circumferentially, the only way to form longitudinal or axial cracks is through rapid heating and cooling.<\/p>\n

Also, consider that the fatigue strength of any steel is less than only 50% of its normal \u2018yield strength\u2019 and that hardened steel is much more brittle than tempered steel. When subjected to excessive fatigue loading, the maximum expected life of any steel is less than three million cycles of alternating load. In terms of a spinning wireline drill rod, this represents less than a few days operation\u2014at most. This same material limitation is behind fatigue failures in drill rod joints when subjected to excessive deviation (refer to our earlier article, \u201cDrill Rod Bending Capabilities and deviated hole applications<\/a>).<\/p>\n<\/div>

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These limitations hold true for all wireline drilling industry carbon and alloy steel grades, all tube forming processes, all parallel and variable wall tubing, and all heat treatment configurations. This is because the transformation temperature is determined by carbon content, and does not worsen significantly without abnormally high levels of carbon (for example, the grade AISI\/SAE 1541 has excessive carbon and a reduced transformation temperature, which was a common grade before heat treatment became popular). Also, whether a drill rod was initially heat treated or not, steel will transform when frictionally heated above its critical temperature, regardless of previous material properties.<\/p>\n<\/div><\/div>

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