Rapid failures of tubing in sour condensate wells have led to an investigation of possible causes of these failures. The presence of hydrogen sulfide points to sulfide stress corrosion cracking as the cause. The effect of untempered martensite was studied as to its bearing on the susceptibility of steel to sulfide stress corrosion cracking.
Test bars from API Grade N-80 tubing previously annealed were heated into the austenitizing range, furnace cooled to temperatures above, within, and below the transformation range, and then water quenched. Over this range of pre-quenching temperatures, samples were produced having from 0 percent to 100 percent martensite. After machining to final size, test bars were two-point beam loaded, with a constant moment between loading points, and immersed in a bath of distilled water saturated with hydrogen sulfide and carbon dioxide. All samples were loaded to various stress levels below the yield strength.
The susceptibility of steel of the chemical composition commonly used in API Grade N-80 pipe (80,000 psi minimum yield strength) to sulfide stress corrosion cracking under these conditions was found to fall into three categories, depending on the percentage of martensite present.
From 0 percent martensite to between 30 percent and 35 percent martensite, the steel is not susceptible.
From 30 percent to 35 percent up to 75 percent to 80 percent martensite, the permissible applied stress to avoid failure decreases with increase of this phase.
Over 75 percent to 80 percent martensite, apparently any applied stress will cause failure.
It was found that when martensite begins to show evidence of continuity in the microstructure, the steel becomes susceptible to sulfide stress corrosion cracking.
