HARDNESS MEASUREMENTS

Hardness testing of ferritic heater tubes can indicate that the tubes experienced a severe overheat or identify some forms of embrittlement or carburization. A severe overheat exceeding the lower critical transformation temperature of the metal can cause a hard microstructure to result if the cooling rate is fast enough. If the cooling rate is slow, softening may actually occur. The hard tubes can be susceptible to brittle fracture if they are mishandled or impacted. They can also be susceptible to some forms of stress cracking like sulfide stress cracking during downtime. Refer to API Std 530 for a listing of material transformation temperatures.

Hardness testing of ferritic heater tubes can also be used to qualitatively determine how evenly fired are the tubes. In Cr-Mo tubes, thermal softening will occur at elevated temperatures. Tubes in the firebox which are softest may represent the hottest tubes. Softening can indicate a reduction in tensile strength for a ferritic material.

885°F embrittlement and external carburization may be identified through hardness changes. With each mechanism, there will be a noticeable increase in hardness. However, if the depth of carburization is not significant, the field test instruments may not identify a change and measure the base metal hardness below the hardened layer.

Hardness measurements can be made using available sonic and impact field testing instruments. Caution should be taken when obtaining hardness measurements to assure adequate surface preparation. Surface roughness and oxide scale can dramatically affect the hardness value.

In general, tubes with hardness outside the normal range (either excessive hardness or excessive softening) should be evaluated for continued service and appropriate repairs made if necessary. This evaluation may require the review of one knowledge of the service and potential deterioration mechanisms that may result.