NACE is the National Association of Corrosion Engineers, NACE publishes many documents related to corrosion control including NACE MR0175 which provides the requirements for metallic materials exposed to H2S in oil and gas production environments.

NACE MR0175 lists the requirements for carbon steels, low alloys and corrosion resistant alloys.  For each material group, NACE MR0175 provides the following metallurgical requirements:

Chemistry

Hardness

Heat treatment

HIC resistance etc

NACE MR0175 also provides the environment limits i.e, amount of H2S expressed as partial pressure, the maximum chloride content, pH range etc.

A NACE MR0175 compliant material (sometimes inaccurately referred to as NACE material or NACE piping) is a material that meets all the requirements of NACE MR0175 and can be used in H2S environments within the limits set by the standard.

It is extremely important to say: Hardness below 22HRC is required for some materials but it is not the only NACE MR0175 requirement