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Definition: Hardness testing is a mechanical testing method for determining the resistance of a material to the penetration of an indenter. Depending on the method, the hardness is calculated from the size of the indentation or the indentation depth. Common standards are DIN EN ISO 6506 (Brinell), 6507 (Vickers) and 6508 (Rockwell).

Practical relevance: Hardness is an indirect parameter for strength, wear resistance and heat treatment condition. It is suitable for incoming goods inspections, weld seam assessments and component checks. Influencing factors are the test force, dwell time, surface condition and microstructure inhomogeneity. Permissible hardness ranges are bindingly specified in many material standards.

Decision-making perspectives:

• Technical decision-makers: Checking of heat treatment, case hardening or microstructure homogeneity.
• Purchasing/project management: Specification of permissible hardness values in orders and acceptance criteria.
• Science: Correlation between hardness, microstructure and tensile strength.
• Insurance/law: Documented verification of material properties in the event of quality deviations.

Typical testing or verification methods: Vickers hardness testing (HV), Brinell hardness testing (HBW), Rockwell hardness testing (HRC, HRB), portable hardness testing.

FAQ:

• Why is hardness often tested instead of tensile strength?
• Hardness testing is faster, usually minimally destructive and allows an indirect estimate of the strength.

Definition: High-temperature water refers to water at elevated temperatures and pressures, as encountered in power plant, reactor or process facilities. Under these conditions, solubility, electrochemical behaviour and corrosion mechanisms change significantly. Materials may become more prone to oxidation, stress corrosion cracking or hydrogen uptake.

Practical relevance: Typical applications include steam generators, pressurised water reactors or high-pressure boilers. Relevant assessment parameters are temperature (>150 °C), pressure, oxygen content, pH value and conductivity. Normative references can be found, among others, in VGB guidelines or ASME code regulations. Incorrect assessment can lead to wall thickness loss, crack formation or HTHA.

Decision-making perspectives:

• Technical decision-makers: Selection of suitable materials and water chemistry concepts.
• Purchasing/project management: Specification of corrosion-resistant materials and monitoring systems.
• Science: Investigation of electrochemical processes at high temperatures.
• Insurance/law: Assessment of corrosion damage and compliance with technical regulations.

Typical testing or verification methods: Autoclave tests, electrochemical high-temperature measurements, wall thickness measurement (UT), metallography.

FAQ:

• Why is high-temperature water particularly corrosive?
• Elevated temperature and pressure change reaction kinetics and passive layer stability, which can accelerate corrosion processes.

Definition: HTHA (High Temperature Hydrogen Attack) is a hydrogen-induced material degradation that occurs at elevated temperatures and hydrogen partial pressures. Atomic hydrogen reacts with carbides in the steel to form methane, leading to decarburisation and the formation of internal microcracks. The damage is usually not directly visible from the outside.

Practical relevance: Low-alloy steels in refineries, ammonia or hydrocracking plants are particularly affected. The assessment is based on operating parameters (temperature, H₂ partial pressure) in accordance with API 941 (Nelson curves). Metallographic examinations reveal decarburisation zones, intergranular cracking and methane bubbles. Undetected HTHA can lead to brittle component failure.

Decision-making perspectives:

• Technical decision-makers: Material selection, inspection intervals and FFS assessments for hydrogen-carrying plants.
• Purchasing/project management: Specification of suitable Cr-Mo steels or higher-alloyed materials.
• Science: Analysis of hydrogen diffusion, carbide stability and microstructural degradation.
• Insurance/law: Verification of standard-compliant material selection and inspection strategy.

Typical testing or verification methods: Metallography (replica technique), ultrasonic testing (UT), hardness testing, fracture mechanics assessment.

FAQ:

• How is HTHA detected?
• By metallographic microstructure examination, supplemented by ultrasonic testing and assessment in accordance with API 941.