API Spec 6A  and 16A specify four standard material property designations, identified by yield strengths. The following table lists the API Spec 6A material designations and the measured minimum mechanical property requirements.

Table 1 – Designated material property requirements for bodies, bonnets and end and outlet connections.

Material Designation0.2% Yield Strength
Tensile Strength
Elongation in 50 mm (2 in)
Reduction in Area
36K a36,00070,00021No requirement
a 36K material designated for use only for bodies of fabricated equipment when stress levels allow.
b API Spec 16A requires 18 elongation for 75K material.

Although API Spec 6A exactly specifies minimum mechanical properties for end and outlet connections, it allows manufacturers to choose among 4 categories of material strength for bodies and bonnets based on calculated stress levels within the assembled equipment at test pressure. The manufacturer may join one strength of material to another by welding as determined by design requirements.

The table below indicates the common materials used by manufacturers in compliance with requirements.

Table 2 – Designated material requirements for bodies and bonnets.

Pressure Rating (psi)2,0003,0005,00010,00015,00020,000
Material Designation36K, 45K, 60K, 75K36K, 45K, 60K, 75K36K, 45K, 60K, 75K36K, 45K, 60K, 75K45K, 60K, 75K45K, 60K, 75K
a API Spec 6A Top Connectors shall have bodies, bonnets and collars made from 75K material.
b Bodies may have end and outlet connections of higher strength material, made integral by welding, in compliance with the requirements of API Spec 6A.

Equipment connections need not have any higher material strength than that of the lowest pressure connection. For example, manufacturers may make a 7-1/16 15M x 7-1/16 10M Flanged Adapter Spool entirely from 60K material and meet API Spec 6A requirements. This unit of equipment has a 10,000 psi working pressure, as determined by its’ lowest pressure connection, and 60K material meets the minimum mechanical property requirements for 10,000 psi working pressure end connections.

The table below lists the material for integral end and outlet connections of pressure containing equipment.

Table 3 – Designated material requirements for integral end and outlet connections

Flanged and Hubbed

Pressure Rating (psi)2,0003,0005,00010,00015,00020,000
Material Designation60K, 75K60K, 75K60K, 75K60K, 75K75K75K


Pressure Rating (psi)2,0003,0005,00010,00015,00020,000
Material Designation45K45K45K

The table below lists the material requirements for API 6A specified weld neck flanges.

Table 4 – Designated material requirements for weld neck flanges.

6B Weld Neck Flange

Pressure Rating (psi)2,0003,0005,00010,00015,00020,000
Material Designation45K45K45K

6BX Weld Neck Flange

Pressure Rating (psi)2,0003,0005,00010,00015,00020,000
Material Designation60K, 75K60K, 75K60K, 75K60K, 75K75K75K

Commonly, manufacturers use the commercial steels shown in the table below to make API pressure control equipment. NACE specifies all of these materials as satisfactory for Hydrogen Sulfide Service (H2S) per MR0175 provided the hardness does not exceed 22 Rockwell C (22 RC), equivalent to 237 Brinell Hardness (237 HBW) per ASTM E140.

Table 5 – Commercial steels used for pressure control equipment

Material aAPI Min. Yield/Min. HBWbNACE max. HBW
A106 Carbon Steel36K/140237
A105 Carbon Steel45K/140237
1040 Low Alloy Steel c60K/174237
4130 Low Alloy Steel60K/174,   75K/197 d237
4140 Low Alloy Steel60K/174,   75K/197 d237
8630 Low Alloy Steel60K/174,   75K/197 d237
410 Martensitic Stainless Steel60K/174,   75K/197 d237
a For special purposes manufacturers may use material other than the ones shown in this table. In most cases the NACE maximum hardness still applies for ferrous magnetic steels. Operators should consult with the equipment manufacturer concerning acceptability of higher hardnesses on specific materials.

b Abbreviation for Brinell Hardness taken using a carbide indenter.

c Typically 6B, 5000 psi and lower pressure connections.

d 75K material can substitute for 60K material

API and NACE both specify material chemistry limitations; otherwise NACE specifies only upper hardness limits for specific materials. API additionally specifies: tensile strength, yield strength, elongation, and reduction of area; all confirmed by destructive testing of a specific test sample (referred to as a Qualification Test Coupon or Test Coupon) taken from the same heat of steel and processed in the same manner as the production part(s) it represents. The hardness test(s) taken on this sample provide(s) a reference to compare to production parts processed in the same way. The hardness test in this case provides a quality control measurement to give the manufacturer “confidence” that the “same processes” gave the same result.

Caution: API Spec 6A requires design stress limits that the manufacturer must consider when choosing the strength of material for bodies. Operators should consult with the equipment manufacturer when questions arise concerning justification of acceptance of the Brinell Hardness of a body that falls below that required for the lowest pressure connector integral to that body. 

Brinell Hardness has no bearing on material suitability for “low temperature” service.

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