It is generally recognised that the risk of sulphide stress corrosion cracking in ferritic steels increases with hardness, and maximum hardness limits are the basis of guidelines for avoidance of cracking in many standards. It is also clear that higher hardnesses are tolerable in less severe environments. This project has investigated the effects of environments containing low levels of H2S on welded ferritic steels. The concern is over environments which come just above the NACE MR01751 limit of 0.05psi partial pressure of H2S. Accordingly, tests have been performed, principally using environments with up to ten times this partial pressure, and also in the presence of different partial pressures of CO2. Some tests were performed using higher levels of H2S. Threshold hardness levels show a clear variation with CO2 partial pressure, and with H2S partial pressure over the range 0.05 to 2psi. In many mildly sour service situations, at or near the NACE MR0175-98 H2S limit, relaxation of hardness limits is reasonable. Maximum permissible hardness levels for welded steels directly exposed to mildly sour service environments have been presented as a function of H2S partial pressure and pH, and are applicable for total pressures of up to 2000psi (138 bar).