Hardness control for Sour services
This article provides quick references for hardness test requirements in sour service applications of Carbon steel, low alloy steel.
Hardness control is one of the numerous methods used to prevent S-SCC (Sulphide stress corrosion cracking) in sour service and Hydrogen induced cracking in general. Metal susceptibility to S-SSC closely related to material strength (as indicated by hardness), which is affected by chemical composition, heat treatment, fabrication method, and microstructure present in the material.
Standards for Hardness Testing & hardness requirements:
ISO 6506-1, Brinell hardness test, Part 1: Test method
ISO 6507-1, Vickers hardness test, Part 1: Test method
ISO 6508-1, Rockwell hardness test, Part 1: Test method
EFC 16, Guidelines on Materials Requirements for Carbon and Low Alloy Steels for H2S-Containing Environments in Oil and Gas Production
NACE MR0175-2, Materials for Use in H2S Containing Environments in Oil and Gas Production, Part 2: Cracking Resistant Carbon and Low Alloy Steels, and the Use of Cast Irons
NACE MR0103, Metallic Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments
NACE SP0472, Methods and Controls to Prevent In-Service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments
NACE Publication 8X194, Materials and Fabrication Practices for New Pressure Vessels Used in Wet H2S Refinery Service
ASTM E140, Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, and Scleroscope Hardness
ISO 18625, Conversion of Hardness Values
Rockwell (HR) hardness test has many scales options with different indenter types and sizes. For carbon steel in sour service, only HRC and HR 15 N are allowed to be used, both are using diamond cone indenter. HRC is standard test used for testing bulk metal while HR 15 N is superficial for testing small and localized area like HAZ. Rockwell is popular because of quick and easy to get the result.
Vickers (HV) hardness test, sometimes called microhardness test only use one type indenter of pyramid shaped diamond with different load. Depend on standards used, HV 30, HV 10, & HV 5 are allowed for test. HV 30 is used for bulk metal, HV 10 is for HAZ, and HV 5 is used for low arc energy welds (<2.5 kJ/mm) which may have very tight HAZ area. Since hardness need to be measured visually by microscope (can be manual or automated by software), detailed sample preparation is needed, but it is also advantage because test can be specific to certain microstructure in the metal and very reliable to find hard spots in HAZ.
Brinell (HBW) hardness test apparatus is using a tungsten carbide ball indenter, normally using a 10 mm diameter and a force of 29.42 kN. Test measurements also need to be measured visually, but the main difference with Vickers is that impression size is significantly bigger, and minimal sample preparation is enough. Brinell is good for measuring bulk material especially when different phases exist but shall not be used to measure HAZ.
Summary of Hardness Requirements
Below table summarize the hardness requirements for weld, HAZ and base metal requirements as per NACE MR0175, NACE MR0103, NACE SP0472 and EFC-16.
Hardness limit per NACE MR0175-2
Hardness limit per EFC 16
Hardness limit per NACE SP0472
General Hardness Control
Higher hardness limit in the weld & HAZ cap for external surface, reflects the less severe hydrogen concentration at the hydrogen exit side. If hydrogen exit from the external surface is impeded (ex: by cathodic protection) then the hardness of cap shall not exceed root limit.
Concern for maximum hardness is that likely it will happen in HAZ or weld metal close to the fusion boundary with highest possible value when using low arc energy or low t8/5 values like fillet welds, in addition, these also give rise to narrow HAZ that may be difficult to sample accurately. Weld metal or HAZ which experience little or no reheating by subsequent passes, in general will be the hardest. When using parent metal with high carbon, concern for migration to weld metal, giving peak hardness just within the weld.
Regions of high hardness are often identified as areas that etch heavily in a standard Nital etch, nevertheless, light etching regions should also be examined, as hard martensitic regions may be resistant to etching. Typically, a reliable way to assess weld area hardness is using a small impression. HAZ hardness impressions shall be entirely within HAZ and located as close as possible to the fusion boundary between the weld metal and HAZ. Weld cap HAZ survey should be positioned so that impressions coincide with the HAZ of the final run or change of profile of the fusion line associated with the final run.
Hardness Conversion Accuracy (HRC or HV?)
There is slight hardness conversion difference between standards for 22 HRC although not very significant, some use 250 HV and 248 HV. This is normal, as explained in ASTM E140, hardness scale conversion is only an approximate process and not possible to state confidence limits for the errors in using a conversion chart. Hardness difference also affected by parameters like composition, properties, heat treatment, etc that maybe slightly different even for similar material. The conversion tables also performed in the past using ASTM test methods in effect at the time of testing, updated standards may affect final results. For example, currently both the Rockwell and Brinell hardness standards allow or require the use of tungsten carbide ball indenters; however, all of the ball scale Rockwell hardness tests and most of the Brinell hardness tests performed to develop these tables used hardened steel ball indenters.
Some fixed-location and portable hardness testers perform internal conversions between hardness scales using the tables in ASTM E140 or ISO 18265. There may also be some instances where hardness scale conversions are handled outside of the standards based on proprietary data or algorithms, especially in some portable instruments where no standardized conversion tables exist. In either case, conversions may be an additional source inaccuracy and uncertainty.