1. Introduction

ASTM A333 Gr.6 is a widely used low-temperature carbon steel pipe material, with a minimum service temperature of -45°C. Due to its excellent low-temperature toughness and cost-effectiveness, it is extensively applied in petrochemical plants, cryogenic equipment, and liquefied gas transmission systems.

In engineering procurement, A333 Gr.6 pipes are often categorized into two supply conditions: "Sour Service" and "Sweet Service" (standard/ordinary). While both types meet the basic chemical composition and mechanical property requirements of the ASTM A333 standard, they differ fundamentally in their resistance to hydrogen sulfide (H₂S) corrosion—a difference that determines their distinctly different application scenarios.

2. Core Differences Between Sour and Sweet Service

2.1 Service Conditions

Sweet Service (Non-sour) : Suitable for environments without H₂S or with extremely low H₂S partial pressure. The primary design considerations are low-temperature toughness and conventional pressure-bearing capacity.

Sour Service : Specifically designed for environments containing wet H₂S, where the material must withstand both low temperatures and H₂S corrosion simultaneously. Sour service materials are also fully acceptable for sweet service applications.

2.2 Chemical Composition Control

Although the base ASTM A333 Gr.6 specification sets limits for C, Mn, P, S, and other elements, sour service pipes require much tighter chemical control:

Parameter	Standard A333 Gr.6	Sour Service A333 Gr.6
Sulfur (S)	≤ 0.035%	Strictly controlled, typically ≤ 0.015%
Phosphorus (P)	≤ 0.035%	Strictly controlled, typically ≤ 0.025%
Carbon Equivalent (CE)	Not mandatory	Tightly limited (e.g., ≤ 0.43%) to prevent excessive hardness

By further reducing impurities such as S and P, and limiting the carbon equivalent, sour service pipes minimize susceptibility to sulfide stress corrosion cracking (SSC) from the chemical composition perspective.

2.3 Hardness and Heat Treatment Requirements

Hardness control is one of the most critical differentiators between sour and sweet service. Sour service A333 Gr.6 requires a maximum hardness of 22 HRC (Rockwell C) or 248 HBW. This is because high-hardness microstructures are highly prone to hydrogen-induced embrittlement and stress corrosion cracking in H₂S environments.

Regarding heat treatment, sour service pipes typically require Normalized or Quenched & Tempered (Q&T) processes to achieve a uniform, fine-grained microstructure that enhances SSC resistance. Standard (sweet) service pipes have relatively relaxed heat treatment requirements.

2.4 Additional Testing Requirements

Sour service pipes must undergo NACE standard compliance verification (e.g., NACE MR0175 / ISO 15156), typically including:

SSC Testing (Sulfide Stress Cracking, NACE TM0177)

HIC Testing (Hydrogen-Induced Cracking, NACE TM0284)

Standard (sweet) service pipes generally only need to meet the low-temperature impact testing requirements of ASTM A333 (-46°C, minimum average impact energy ≥ 20J) along with routine mechanical property verification.

3. Summary and Selection Recommendations

Aspect	Standard A333 Gr.6	Sour Service A333 Gr.6
Service Medium	No H₂S or negligible H₂S	Wet H₂S-containing sour media
Hardness Limit	No special restriction	≤ 22 HRC / 248 HBW
Heat Treatment	Conventional processing	Normalized or Q&T; uniform microstructure
Additional Testing	Low-temperature impact test	SSC + HIC testing per NACE standards
NACE Compliance	Not required	NACE MR0175 compliant

Selection Recommendations: During the design phase, it is essential to determine whether H₂S is present in the service environment. If wet H₂S conditions exist, the procurement specification must clearly state "A333 Gr.6 for Sour Service, compliant with NACE MR0175 / ISO 15156," with explicit requirements for hardness, carbon equivalent, heat treatment, and supplementary testing. Standard A333 Gr.6 pipes should never be used in sour media, as this may lead to catastrophic stress corrosion cracking failures.