Which materials withstand extreme pressures and temperatures over decades without losing their safety properties?
Selecting suitable materials for pressure vessels determines the operational safety, service life and cost-effectiveness of entire plants. This article systematically examines the relevant standards, norms and technical requirements that must be observed in material selection.
From the Pressure Equipment Directive 2014/68/EU through EN 13445 to the proven AD 2000 – here we clearly explain which sets of rules apply and how they are put into practice. The various routes to approval are also presented, from harmonised standards through European Approvals of Materials to particular material appraisals.
- Pressure equipment requires materials that withstand mechanical, thermal and chemical loads throughout their entire service life
- The Pressure Equipment Directive 2014/68/EU forms the binding legal framework for the European market
- EN 13445 and AD 2000 are regarded as recognised technical sets of rules for design and material selection
- Materials must either comply with harmonised standards, hold a European Approval of Materials or be assessed by a particular material appraisal
- Manufacturers bear full responsibility for the suitability of the materials used and their proper documentation
- Correct material selection requires knowledge of the operating conditions, media, pressures and temperatures
- Certification and quality assurance are indispensable components of material selection for safety-relevant plants
Fundamentals of materials science in pressure applications
In pressure applications, the right choice of material determines safety, durability and economical operation. Materials science provides the understanding needed to select and deploy materials in a targeted manner. Every material reacts differently to pressure, temperature and chemical influences.
Knowledge of material properties forms the basis of every responsible design. Engineers must understand how materials behave under different conditions. Only in this way can pressure equipment be developed that meets all requirements.
Technical facilities under pressure and their fields of application
Pressure equipment consists of technical facilities that contain gases, vapours or liquids under a pressure higher than atmospheric pressure. This definition covers a wide range of applications across the most diverse industries. The common denominator: all of them must function with absolute reliability.
In the chemical industry, pressure vessels are used for reaction processes and storage. Power plants use steam boilers to generate energy under high temperatures and pressures. Energy supply depends on pipelines that transport natural gas safely.
Further important fields of application include:
- Autoclaves in the food industry for preservation and sterilisation
- Pressure accumulators in pneumatic systems for automation technology
- Heat exchangers in heating systems for efficient heat transfer
- Medical equipment such as oxygen cylinders and sterilisers
- Hydraulic accumulators in mobile working machines
The fields of application range from energy generation through the process industry to medical technology. Failure of these systems can have serious consequences. This is why the pressure vessel materials used are subject to particular scrutiny.
Material requirements under loading from internal pressure
Materials for pressure equipment must possess exceptional properties. Mechanical strength comes first: the material must withstand the internal pressure without deforming permanently or breaking. But strength alone is not enough.
Ductility plays a decisive role in safety. It describes the ability of a material to deform plastically under load before it fails. This property acts like a built-in safety reserve.
Even under overload, a ductile material does not fracture suddenly. Instead, it shows clear signs of deformation that can be detected in good time. The Pressure Equipment Directive therefore specifically requires an elongation at fracture of at least 14 percent.
Toughness describes how well a material can absorb energy without breaking. This property is especially important under dynamic loads or temperature fluctuations. The required impact energy is at least 27 joules at the lowest operating temperature.
Further central requirements for materials under pressure loading include:
- Chemical resistance to the conveyed fluids and media
- Resistance to ageing over the entire intended service life
- Suitability for the intended processing methods such as welding and forming
- Compatibility when joining different materials
Metallic materials for pressure vessels dominate the field because they meet these requirements optimally. They offer an excellent combination of strength, toughness and processability. The properties must remain constant throughout the intended service life.
Physical and chemical characteristics of pressure equipment materials
The physical properties form the basis for all strength calculations. The yield strength defines the load limit above which a material deforms permanently. The tensile strength indicates the maximum stress a material can withstand before it breaks.
The modulus of elasticity describes the stiffness of a material. It shows how strongly a material deforms elastically under load. These characteristic values must be known for the respective operating temperature.
Thermal properties become important in the case of temperature changes. Thermal conductivity influences how quickly temperature changes spread through the material. The coefficient of thermal expansion determines how strongly the material expands when heated.
Differing coefficients of expansion can cause stresses when joining different materials. These must be taken into account during design. Otherwise leaks or cracks at the joints may occur.
The chemical properties determine long-term stability. Many pieces of pressure equipment come into contact with aggressive media – acids, alkalis, salt solutions or corrosive gases. The chosen material must be permanently resistant to these substances.
Corrosion can gradually reduce the wall thickness. It causes cracks or considerably worsens the mechanical properties. Corrosion resistance depends on the chemical composition of the material.
High-alloy stainless steels contain chromium, nickel and molybdenum for better corrosion protection. These elements form protective oxide layers on the surface. Resistance to ageing is equally critical.
Some materials become brittle over time or lose strength. Thermal influences can change the structure. A good pressure equipment material keeps its properties stable over decades.
Processability is one of the practical basic requirements. Materials must be capable of being welded, formed and heat-treated. No critical changes may occur in the microstructure in the process.
For welded joints of different materials, compatibility is decisive. Different electrochemical potentials can lead to contact corrosion. Different strength values create stress concentrations at the transitions.
Careful selection of suitable pressure vessel materials takes all these factors into account. Only in this way is pressure equipment created that operates safely, economically and durably. Materials science provides the tool for this demanding task.
Materials for pressure equipment: overview of material groups
In the pressure equipment industry, various material groups have become established, each optimised for specific application scenarios. The range extends from economical standard solutions to highly developed materials for extreme operating conditions. Each material group brings characteristic properties that enable targeted selection for different requirement profiles.
The suitability of a material must be ensured for the entire intended service life of the pressure equipment. Special provisions apply to pressure-bearing parts with regard to ductility, toughness and resistance to brittle fracture. These requirements ensure that components also function safely under unfavourable conditions.
Metallic materials for pressure equipment
Metallic materials form the foundation of pressure equipment construction. Unalloyed structural steels represent the basic class and are suitable for moderate pressure ranges at normal temperatures. These materials are convincing thanks to their excellent weldability and economical availability.
Low-alloy steels significantly broaden the range of applications. By targeted addition of alloying elements such as manganese, silicon or chromium in small quantities, strength and toughness improve noticeably. Normalised or normalising-rolled variants offer homogeneous microstructures with reliable mechanical properties.
Ferritic steels are subject to specific calculation rules. The permissible stress values amount to two thirds of Re,t and five twelfths of Rm,20. These values ensure adequate safety reserves for pressure-bearing components.
Fine-grain structural steels are characterised by particularly small grain sizes in the microstructure. This noticeably increases toughness and offers important advantages at low temperatures or under dynamic loads. High-strength structural steels enable slimmer designs through increased permissible stresses.
Austenitic steels form a special material group with outstanding properties. Their characteristic feature is the austenitic microstructure, stabilised by high chromium and nickel contents. These materials offer excellent corrosion resistance and remain tough even at very low temperatures.
For austenitic steel with an elongation at fracture above 35 percent, more generous calculation values apply. The permissible stress reaches five sixths of Re,t and one third of Rm,t. This allows more efficient designs while maintaining the same level of safety.
Unalloyed cast steel enables complex geometries that could not be realised by forging or rolling. The permissible stresses are ten nineteenths of Re,t and one third of Rm,20. Cast materials often exhibit somewhat lower ductility than wrought materials.
Aluminium alloys also find use in pressure equipment. They offer considerable weight advantages and good corrosion resistance. Their use focuses mainly on applications with lower pressures and temperatures.
Special alloys for pressure applications
Special alloys for pressure applications are used under particularly demanding conditions. These high-performance materials enable applications in which conventional materials would reach their limits. The development of such alloys requires extensive metallurgical know-how.
Duplex steels combine ferritic and austenitic microstructure fractions in balanced proportions. This microstructure unites high strength with excellent corrosion resistance. Typical fields of application include offshore applications, seawater desalination and aggressive chemical processes.
Nickel alloys such as Inconel, Hastelloy or Monel offer exceptional resistance to extreme corrosion. These special alloys for pressure applications withstand high temperatures and specific chemical attacks. Their use is indispensable wherever standard steels would fail.
Titanium alloys score with an outstanding strength-to-weight ratio. Their excellent corrosion resistance makes them attractive for demanding environments. However, they require special processing techniques and entail higher material costs.
Copper alloys are chosen for special heat transfer tasks. Their thermal properties predestine them for certain chemical environments. Selection is based on media compatibility and operating temperatures.
High-pressure materials with special properties
High-pressure materials are developed specifically for applications with extreme loads. These materials must safely master exceptional pressures, temperatures or combined stresses. Their development is based on advanced metallurgical concepts.
Quenched and tempered steels undergo special heat treatments such as hardening and tempering. These processes produce optimal combinations of strength and toughness. The resulting microstructure is designed for the highest mechanical load-bearing capacity.
Creep-resistant steels retain their mechanical properties even at elevated temperatures over long periods. These high-pressure materials are indispensable for steam boilers, power plant components and petrochemical plants. Creep rupture strength values play a central role under creep loading.
Low-alloy chromium-molybdenum steels have proven themselves for high-temperature applications. Long-term loads at high temperatures require materials with excellent creep strength. More highly alloyed variants extend the usable temperature range further.
For low-temperature applications, materials with special properties are required. Liquefied gas storage tanks or cryogenic systems require materials that remain sufficiently tough even at minus 196 degrees Celsius. Certain austenitic steels and nickel-based alloys reliably meet these requirements.
Material selection always requires a careful technical assessment. Blanket recommendations fall short, because every application places individual requirements. Factors such as operating pressure, temperature range, medium and expected service life must be considered holistically.
Normative sets of rules and standards for pressure equipment materials
The legal and technical sets of rules for pressure equipment materials form a structured system that supports manufacturers in material selection. These standards and directives ensure a high level of safety across national borders. They create clear framework conditions for placing pressure equipment on the market and operating it.
The normative structure may initially seem complex. However, it offers comprehensible routes to conformity. Different levels complement one another – from European requirements to national specifications.
Material selection for pressure equipment: decision criteria
In selecting materials for pressure equipment, manufacturers face the challenge of weighing up numerous technical parameters. Responsibility for this decision lies solely with the manufacturer. A well-founded choice of material is based on a thorough hazard analysis.
The selection process takes into account the intended use and the planned service life. Every piece of pressure equipment places individual requirements on the material used. A systematic assessment of all relevant criteria leads to safe and economical solutions.
Material selection covers three main areas: mechanical properties, operational framework conditions and media compatibility. These areas influence one another and must be considered holistically. Only a comprehensive analysis guarantees the right choice of material.
Mechanical characteristic values and strength properties
The mechanical characteristic values form the foundation of every material selection for pressure equipment. They determine the load-bearing capacity and safety of the entire system. Without precise knowledge of these values, no reliable calculation is possible.
The yield strength characterises the stress above which a material deforms permanently. For steels without a pronounced yield point, the 0.2 percent proof strength is used. Austenitic steels often require the specification of the 1.0 percent proof strength.
The tensile strength indicates the maximum load-bearing capacity before material failure. This value is temperature-dependent and must be known for every operating temperature. Higher temperatures generally lead to lower strength values.
Essential strength properties include the following characteristic values:
- Modulus of elasticity for deformation calculations and stiffness analyses
- Fatigue strength under dynamic loads and fatigue loading
- Notched-bar impact toughness for behaviour under impact loading
- Creep rupture strength for high-temperature applications over long periods
- Creep strength for long-term loads at elevated temperatures
Adequate ductility represents a critical safety factor. Materials for pressure equipment must exhibit an elongation at fracture of at least 14 percent. The impact energy should not fall below 27 joules.
These minimum values guarantee tough material behaviour without the risk of brittle fracture. Brittle materials can fail suddenly under unforeseen load peaks. Ductile materials, by contrast, show visible deformations as advance warning.
In high-temperature applications, creep rupture strength values gain particular importance. They describe the sustainable stress over periods of 100,000 hours and more. Creep processes lead to gradual deformations that can result in failure in the long term.
The manufacturer must establish all required characteristic values appropriately. Normative sets of rules provide orientation for standardised materials. Special materials require additional tests and conservative estimates.
Operating conditions as a selection criterion
The operating conditions influence material selection for pressure equipment to a considerable extent. Every application places specific requirements on the material. A precise analysis of the actual operating parameters is indispensable.
The design pressure defines the mechanical loading of the material. Higher pressures require either stronger materials or greater wall thicknesses. The pressure rating also determines the required scope of testing and the safety concepts.
The operating temperature fundamentally influences all mechanical properties. Low temperatures considerably increase the risk of brittle fracture. High temperatures reduce strength values and activate creep processes.
Temperature changes generate thermal stresses due to differing coefficients of expansion. Frequent temperature cycles can lead to fatigue phenomena. Resistance to thermal cycling thus becomes an important selection criterion.
Important operational influencing factors include:
- Continuous operation versus frequent start-up and shut-down procedures
- Planned service life from a few years to several decades
- Environmental conditions such as outdoor installation with weathering effects
- Additional mechanical loads from pipelines or support structures
- Aggressive atmospheres in the surroundings of the pressure equipment
The intended service life plays a central role in the choice of material. Temporary test plants place different requirements than permanent production plants. Pressure vessels designed for a 30-year operating life require particularly ageing-resistant materials.
Frequent load changes require materials with high fatigue strength. Every pressure cycle contributes to material fatigue. The permissible number of load cycles must be taken into account in the design.
Environmental conditions can decisively influence the choice of material. Coastal locations with salty air require increased corrosion resistance. Chemical industrial plants with aggressive vapours place additional requirements.
The mode of operation must be fully incorporated into material selection. All foreseeable loading scenarios should be analysed. Exceptional operating states such as emergency shutdowns must also be taken into account.
Media compatibility and corrosion resistance
Media compatibility determines the long-term safety of many pieces of pressure equipment. Corrosive attacks lead to damage more frequently than mechanical overload. The chemical resistance of the material must be ensured throughout the entire service life.
Different fluids trigger different forms of corrosion. Acids, alkalis and salt solutions attack metals in specific ways. Organic solvents or aggressive gases require special materials.
General corrosion removes material uniformly and can be calculated through corrosion allowances. The rate of removal is given in millimetres per year. This value feeds directly into the wall thickness calculation.
Dangerous local forms of corrosion require particular attention:
- Pitting corrosion penetrates the wall locally and is difficult to predict
- Crevice corrosion occurs in narrow gaps with stagnant medium
- Stress corrosion cracking combines mechanical tensile stresses with a corrosive environment
- Hydrogen-induced cracking affects certain steels in an H₂-containing atmosphere
- Intergranular corrosion damages improperly heat-treated austenitic steels
Corrosion resistance must be verified for every material-medium combination. Corrosion tables provide orientation for common combinations. Empirical values from operating practice complement the theoretical knowledge.
For unknown or critical media, laboratory tests are advisable. Exposure tests under realistic conditions provide reliable data. The test duration should be chosen as long as possible.
Stainless austenitic steels show good resistance to many media. However, their universal applicability is limited. Chloride-containing solutions can also attack high-alloy steels.
For special requirements, special alloys are available. Nickel-based alloys offer extreme corrosion resistance. Titanium and tantalum are suitable for particularly aggressive media.
Coatings or linings extend the application possibilities of economical base materials. Enamel coatings protect steel against chemical attacks. Plastic linings are suitable for low pressures and temperatures.
Ageing resistance ensures consistent properties over the service life. Some steels become brittle at certain temperatures. Precipitation processes can change strength values.
UV radiation, oxidation and thermal cycles accelerate ageing processes. These influences must be taken into account in outdoor installations. Regular inspections monitor the condition of the material.
Processability must be ensured in material selection for pressure equipment. Weldability without cracking or embrittlement is essential. Forming operations such as bending or deep drawing must be possible.
Material combinations require particular attention. Different materials at weld seams must be compatible. Electrochemical corrosion through contact of less noble metals must be avoided.
Pressure equipment safety through suitable materials
Systematic testing procedures and continuous documentation form the foundation of pressure equipment safety. The selection of suitable materials alone is not sufficient. Only through consistent quality assurance across the entire manufacturing process does a reliable piece of pressure equipment come into being.
The right choice of material considerably minimises risks. Combined with proper processing and thorough testing, it ensures safe operation throughout the intended service life.
Quality assurance and materials engineering test procedures
Quality assurance begins as early as material procurement. The manufacturer must ensure that the ordered materials meet the specified requirements. This is done through inspection documents from the material manufacturer.
For less critical components, a simple certificate may be sufficient. The inspection certificate 2.1 in accordance with DIN EN 10204 confirms that the material complies with the order.
Important pressure-bearing parts of categories II, III and IV require higher levels of verification. Here a certificate with specific inspection is used – typically a 3.1 certificate. In this case, tests are carried out on the specific material delivered.
The tests carried out include:
- Tensile tests to determine yield strength and tensile strength
- Charpy notched-bar impact bending tests for toughness inspections
- Chemical analyses by spectroscopy
- Hardness tests to check strength properties
The determined values are documented and are traceable to the respective batch or heat number. In particularly critical cases, a 3.2 certificate is required. In this case, a representative of the purchaser or an independent inspector is additionally present at the acceptance tests.
A certified quality management system of the material manufacturer creates additional confidence. When the manufacturer applies a system in accordance with ISO 9001 that has been specifically assessed for material production, this increases reliability. Notified bodies check the processes for melt control, heat treatment and documentation in the process.
During manufacture, non-destructive testing procedures are used. These methods detect defects without damaging the material:
- Ultrasonic tests detect internal defects in plates and tubes
- Radiographic tests using X-rays or gamma rays make weld seam defects visible
- Magnetic particle tests detect surface cracks in ferromagnetic materials
- Penetrant tests reveal the finest surface defects in all materials
Metallographic examinations complement the testing programme. They show the microstructure and can reveal defects such as inclusions, segregations or undesirable phases. Corrosion tests simulate resistance to specific media.
The traceability of the materials must be ensured throughout the entire manufacturing process. From the incoming raw material through all processing steps to the finished pressure equipment, it must be possible to trace which material was installed where. This is done through marking – for example stamped numbers, colour markings or accompanying documentation.
If identification is lost, the material can no longer be used. The records are then missing, and conformity can no longer be demonstrated.
Materials engineering in practice
Materials engineering combines theoretical knowledge of material properties with practical manufacturing processes. This discipline plans and monitors all steps that can influence material properties.
Welding locally changes the microstructure through melting and rapid cooling. In the heat-affected zone, hardness peaks or grain coarsening can occur. These changes reduce toughness.
Suitable measures control these effects:
- Selection of suitable welding filler materials
- Controlled heat management with preheating
- Monitoring of the interpass temperature
- Subsequent heat treatments for stress-relief annealing
Forming operations such as cold bending can strengthen materials and locally embrittle them. If material properties could be strongly altered by manufacturing methods, an appropriate heat treatment must be carried out. Where necessary, intermediate annealing or final annealing is required.
Heat treatments specifically change the microstructure. Normalising, quenching and tempering or solution annealing enable the desired combinations of properties. Materials engineering plans these processes so that all requirements are met in the end.
Practical implementation requires close cooperation between various specialist departments. Design, manufacturing and quality assurance must work hand in hand. Only in this way can the materials engineering specifications be reliably observed.
Risk minimisation through the right choice of material
Many cases of damage can be traced back to unsuitable materials. Brittle fractures at low temperatures, stress corrosion cracking in chloride-containing media or creep failure at high temperature are typical examples.
Through careful hazard analysis, such risks can be identified. Important questions in this regard are: what happens if the temperature drops unexpectedly? Which emergency situations can occur? How does the material behave in the event of deviations from normal operation?
By playing through such scenarios, suitable materials can be selected. These also offer adequate safety reserves under unfavourable conditions.
Safety factors in the calculation rules take this into account. They set the permissible stresses well below the strength limits. This creates a buffer that can absorb unforeseen loads.
The combination of several safety elements sustainably minimises risks:
- Suitable materials with adequate safety reserves
- Standard-compliant design in accordance with recognised sets of rules
- Careful manufacturing with qualified procedures
- Seamless quality assurance and documentation
- Regular recurring inspections during operation
Recurring inspections during operation continuously monitor the condition. They detect any changes at an early stage and enable timely countermeasures.
Materials alone do not create safety. Only the interplay of the right selection, proper processing, thorough testing and responsible operation ensures pressure equipment safety. This holistic approach ensures that pressure equipment fulfils its task reliably and without endangerment.
The continuous improvement of processes contributes to long-term safety. Experiences from operation feed into the further development of materials and testing procedures. In this way, a closed loop emerges that steadily increases reliability.
Harmonised standards, European Approvals of Materials and particular material appraisals are available as proven routes to approval. The technical sets of rules AD 2000 and EN 13445 are regarded as the recognised state of the art. They offer manufacturers clear orientation in calculation, design and manufacture.
Responsibility for material selection lies with the pressure equipment manufacturer. A comprehensive hazard analysis takes into account all relevant operating conditions. Mechanical characteristic values, media compatibility and corrosion resistance determine the final material decision.
Quality assurance runs through the entire process from material procurement to final inspection. Only through a systematic approach can risks such as brittle fracture or creep failure be minimised. The right choice of material creates the necessary safety reserves for unforeseen situations.
The variety of available materials for pressure equipment makes it possible to find a suitable solution for every requirement. The continuous further development of materials and testing procedures contributes to increasing safety.
