Heat Resistant Steels Sourced numerical data plus engineering analysis

Material / Grade

Therma 347H

Therma 347H is a Heat Resistant Steels grade centred on retention of oxidation resistance and mechanical stability at elevated temperature. This dossier separates sourced values from engineering guidance and comparison indices.

Hardness potential39 / 100Comparison index

Toughness79 / 100Comparison index

Impact resistance78 / 100Comparison index

Bending resistance80 / 100Comparison index

Abrasive wear40 / 100Comparison index

Steel Overview

Complete introduction and metallurgical analysis

Therma 347H belongs to Heat Resistant Steels and selection centres on retention of oxidation resistance and mechanical stability at elevated temperature.

The principal recorded elements are C≈0.05%, Cr≈17.5%, Ni≈9.5%, Si≈0.4%. Mid-range values support engineering interpretation and do not replace purchase limits.

The expected microstructure is austenitic or ferritic heat-resistant matrix whose scale adhesion and phase stability depend on composition and exposure; delivery condition, section size, melt quality and processing history can change that state.

Comparison indices are hardness 39/100, toughness 79/100, impact 78/100, wear 40/100 and bending 80/100. These are internal indices, not standard test results.

The screening temperature range is 700–850 °C; strength or hardness loss, oxidation, exposure time and creep must be assessed separately.

A representative process route is solution annealing or grade-specific heat treatment followed by clean fabrication and controlled high-temperature service. Obtain actual temperatures, times and cooling media from the grade and product data sheet.

Manufacturing indices are weldability 72/100, machinability 29/100, corrosion resistance 65/100 and fatigue resistance 65/100; use them for screening only.

The principal risk is creep, sigma or secondary-phase embrittlement, oxidation or rapid strength loss outside the qualified temperature range. Evaluate geometry, surface condition, environment and the governing failure mechanism together.

A similar name or calculated relationship is not proof of interchangeability; match chemistry, specification, condition, heat treatment and MTC.

Engineering conclusion: Therma 347H is a rational candidate when improved resistance to scaling, hot corrosion and thermal exposure matches the real load, environment and certified product condition.

Recorded standards:ASTM/AISI 347HUNS S34709

International Equivalents

International equivalents and designations

A close counterpart is not automatically interchangeable. Confirm chemistry, product standard, delivery condition, dimensions and heat treatment.

StandardOfficial designation for the same grade

Therma 347H

A registered identity within the same grade record.

OutokumpuClose counterpart requiring verification

Therma 347H

Match chemistry, product standard, delivery condition, heat treatment and MTC before substitution.

ASTM/AISI TypeClose counterpart requiring verification

347H

Match chemistry, product standard, delivery condition, heat treatment and MTC before substitution.

UNSClose counterpart requiring verification

S34709

Match chemistry, product standard, delivery condition, heat treatment and MTC before substitution.

Chemical Composition

Chemical composition and the role of each element

The principal recorded elements are C≈0.05%, Cr≈17.5%, Ni≈9.5%, Si≈0.4%. Mid-range values support engineering interpretation and do not replace purchase limits. Composition is shown for screening and traceability. The current purchase standard and the heat-specific MTC remain authoritative.

Element	Weight-percent range	Metallurgical role and effect
C	≈ 0.05 %	Carbon raises hardness, strength and carbide potential; excessive carbon can reduce toughness and weldability.
Cr	≈ 17.5 %	Chromium promotes hardenability and carbide formation and, at sufficient levels, corrosion or oxidation resistance.
Ni	≈ 9.5 %	Nickel generally improves toughness and through-section response and stabilises austenite in stainless systems.
Si	≈ 0.4 %	Silicon supports deoxidation and solid-solution strength and can influence tempering or oxidation response.

Outokumpu range datasheet — typical chemical composition, % by mass Sourced with conditions

Engineering Behaviour

Engineering behaviour and selection response

Hardness potential39 / 100

Designed primarily for high-temperature oxidation/creep resistance; HRC is not the primary selection metric. Internal comparison index: 39/100.

Engineering analysis

Toughness79 / 100

Toughness has an internal index of 79/100 (high). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

Impact resistance78 / 100

Impact resistance has an internal index of 78/100 (high). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

Bending resistance80 / 100

Bending resistance has an internal index of 80/100 (high). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

Abrasive wear40 / 100

Abrasive wear has an internal index of 40/100 (low). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

Corrosion resistance65 / 100

Corrosion resistance has an internal index of 65/100 (high). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

Weldability72 / 100

Weldability has an internal index of 72/100 (high). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

Machinability29 / 100

Machinability has an internal index of 29/100 (low). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

High-temperature strength82 / 100

Guidance maximum application temperature is sourced from Therma data; load, time, atmosphere and creep can reduce usable limits. Internal comparison index: 82/100.

Engineering analysis

Creep resistance82 / 100

Creep resistance has an internal index of 82/100 (very high). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

Fatigue resistance65 / 100

Fatigue resistance has an internal index of 65/100 (high). Actual performance requires a documented product condition, heat treatment and test context.

Engineering analysis

Numerical scores are internal comparison indices. They are not standard test results, allowable design stresses or a manufacturer guarantee. Data Method

Hardness & Hardenability

Hardness and hardenability

Designed primarily for high-temperature oxidation/creep resistance; HRC is not the primary selection metric. Internal screening index: 39/100 (low).

39/100

low

Comparison index

Governing factors

Chemistry and section size
Austenitising and quench severity
Tempering temperature and time
Test method and surface preparation

Data limits

Final acceptance requires the current standard, product form, dimensions, delivery condition and heat-specific MTC.

Impact & Toughness

Toughness, impact and fracture behaviour

Impact resistance is indexed at 78/100 and toughness at 79/100. These are not Charpy or Izod energy values; temperature, orientation, geometry and actual microstructure govern component behaviour.

78/100highComparison index

Microstructure, hardness and grain size influence impact tolerance.
Sharp radii, notches and surface defects reduce fracture resistance.
Test temperature, specimen direction and section must be identified.
For critical parts, accept impact energy only from a documented test of the same condition.

Temperature & Creep

Service temperature, thermal stability and creep

Guidance maximum application temperature is sourced from Therma data; load, time, atmosphere and creep can reduce usable limits. 700–850 °C is a screening range or sourced limit according to its evidence label. Check strength loss, oxidation, creep, exposure time and environment.

Continuous service

850 °CSourced with conditions

Review the limit against strength or hardness loss and exposure time.

Intermittent service

Do not assign an intermittent limit without product-specific source data.

Tempering and phase stability

Working and coating temperatures must remain compatible with the tempering condition and required microstructure.

Oxidation and environment

Oxidation or corrosion may become limiting before the strength criterion.

Relative creep resistance82/100Comparison index

The relative creep index is 82/100 (very high). It is not an allowable creep stress or rupture-life value.

Do not select this grade for long-term static high-temperature loading from the internal index alone.

Wear & Surface Behaviour

Wear resistance and surface behaviour

Abrasive wear

The abrasive-wear index is 40/100; final hardness, carbides, microstructure, contact mode and lubrication govern actual resistance.

Comparison index

Adhesive wear

Assess adhesive wear with the counterface material, lubrication, roughness, pressure and temperature.

Engineering analysis

Surface improvement

Evaluate surface treatment or coating only after confirming compatibility with chemistry, substrate and process temperature.

Engineering guidance

Mechanical & Physical Properties

Mechanical and physical properties

The record contains 1 sourced mechanical-property rows; each applies only to its stated condition, dimensions and test temperature.

Property	Value	Condition	Dimension	Test temperature
Maximum application temperature	≈ 850 °C	Air; guidance per EN 10095 / Outokumpu	According to the product data sheet	According to the cited test

Sourced with conditions

Physical properties

Density≈ 7.9 g/cm³Sourced with conditions

Elastic modulus≈ 200 GPaSourced with conditions

Thermal expansion to 600°C≈ 18.8 ×10⁻⁶/KSourced with conditions

Thermal conductivity≈ 15 W/m·KSourced with conditions

Heat capacity≈ 500 J/kg·KSourced with conditions

Electrical resistivity≈ 0.73 Ω·mm²/mSourced with conditions

Heat Treatment

Heat-treatment stages and process controls

The representative family route is solution annealing or grade-specific heat treatment followed by clean fabrication and controlled high-temperature service. Numerical temperatures are shown only when supported by the record.

Annealing / stress relief

Engineering guidance

Assess the supply condition, residual stress, section size and prior machining before selecting time and temperature.

Cooling: Grade- and process-specific cooling

Annealing / stress relief

Engineering guidance

Assess the supply condition, residual stress, section size and prior machining before selecting time and temperature.

Cooling: Grade- and process-specific cooling

Preheating and austenitising

Engineering guidance

Set preheat steps and austenitising parameters from the grade/product data sheet and actual section size.

Cooling: Grade- and process-specific cooling

Quenching / controlled cooling

Engineering guidance

Choose the quench medium and cooling severity against hardenability, geometry, distortion and crack risk.

Cooling: Grade- and process-specific cooling

Tempering / property adjustment

Engineering guidance

Set tempering or ageing to the required hardness, toughness and service-temperature balance.

Cooling: Grade- and process-specific cooling

Tempering / property adjustment

Engineering guidance

Set tempering or ageing to the required hardness, toughness and service-temperature balance.

Cooling: Grade- and process-specific cooling

Process control: Review delivery condition, actual section, equipment capability, crack and distortion risk and target hardness before approving the cycle.

Engineering Charts

Hardness, toughness, impact, temperature and process charts

Chemistry data retain their evidence label; 0–100 charts are internal indices and stages without temperature are process sequences.

Multi-axis engineering profile

Comparison index

Chemical composition

Derived from source data

Hardness–toughness balance

Comparison index

Toughness, impact and fatigue

Comparison index

Wear and surface response

Comparison index

Mechanical properties or comparison indices

Sourced with conditions

Service temperature

Sourced with conditions

Heat treatment

Engineering guidance

Related-grade similarity

Comparison index

All comparison indices

Comparison index

Failure Modes & Selection

Potential failure modes and selection guidance

Potential failure mechanisms

creep, sigma or secondary-phase embrittlement, oxidation or rapid strength loss outside the qualified temperature range.
Fatigue initiation from surface defects, section transitions or stress concentration.
Property loss after unsuitable heat treatment, cooling or finishing.
Corrosion, oxidation or environmental attack when protection is inadequate.

Selection and design notes

Match designation, material number, product standard and supply form before purchase.
Compare the governing failure mechanism with hardness 39, toughness 79, impact 78 and wear 40 indices out of 100.
Use mechanical values only within their cited condition, dimensions and test temperature.
Confirm the heat-treatment route and target hardness with the processor before final machining.
Review the heat number, MTC and required test results before release.

Derived calculations

Pitting-resistance equivalent PREN (indicative)17.5 Calculated

Calculated from recorded inputs; apply only within the stated formula scope and limitations.

Advantages & Limitations

Advantages, limitations and unsuitable applications

Advantages

improved resistance to scaling, hot corrosion and thermal exposure.
A clearly labelled engineering profile supports comparison with related grades.
Grade identity, chemistry, counterparts and references remain traceable in one dossier.

Limitations and weaknesses

creep, sigma or secondary-phase embrittlement, oxidation or rapid strength loss outside the qualified temperature range.
A 0–100 index cannot be converted into design stress, impact energy or creep life.
Hardness and toughness depend on section, delivery condition and actual processing.
Do not approve a substitute before checking the product standard and heat-specific MTC.

Cases requiring caution or an alternative material

Critical design without condition- and section-specific test dataDirect substitution based only on name or similarity scoreLong-term creep service without sourced time–stress–temperature datacreep, sigma or secondary-phase embrittlement, oxidation or rapid strength loss outside the qualified temperature range

Applications & Processing

Industrial applications and processing

Typical applications

Furnace, heat-treatment and high-temperature process components
Exhaust and turbocharger components
Valves, pumps and flanges
Plate, sheet and tubular products to the applicable product standard
furnace and heat-treatment equipment
burner, radiant and exhaust components
high-temperature process hardware
oxidation-resistant sheet, plate and fabricated parts

Manufacture, welding and surface engineering

Weldability has an internal index of 72/100 (high). Actual performance requires a documented product condition, heat treatment and test context.

Forming and forging depend on steel family, delivery condition and the grade-specific qualified temperature range.

Select nitriding, induction hardening or coating against chemistry, substrate hardness and application.
Keep coating-deposition temperature compatible with the tempering condition and substrate.
Control hydrogen embrittlement, residual stress and distortion in high-strength components.

Engineering Conclusion

Engineering conclusion and selection recommendation

Therma 347H is worth evaluating when the principal need is improved resistance to scaling, hot corrosion and thermal exposure. Internal indices of hardness 39/100, toughness 79/100, impact 78/100 and wear 40/100 must be aligned with the real failure mechanism. The leading risk is creep, sigma or secondary-phase embrittlement, oxidation or rapid strength loss outside the qualified temperature range. Final approval requires the current standard, product form, dimensions, delivery condition, heat treatment and heat-specific MTC.

Engineering analysis

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Frequently Asked Questions

Frequently asked questions

What is Therma 347H steel?

What are the main applications of Therma 347H?

Application contexts include Furnace, heat-treatment and high-temperature process components, Exhaust and turbocharger components, Valves, pumps and flanges and Plate, sheet and tubular products to the applicable product standard, subject to the product standard and actual condition.

How hard can Therma 347H be?

Designed primarily for high-temperature oxidation/creep resistance; HRC is not the primary selection metric. Internal screening index: 39/100 (low).

Is Therma 347H suitable for impact loading?

Impact resistance is indexed at 78/100 and toughness at 79/100. These are not Charpy or Izod energy values; temperature, orientation, geometry and actual microstructure govern component behaviour.

What is the service-temperature limit of Therma 347H?

Can Therma 347H be replaced by a close grade?

Approve substitution only after matching chemistry, specification, product form, condition, heat treatment and MTC.

Steel Comparison

Engineering steel comparison

Compare this grade with any other dossier. Scores from 0 to 100 are internal screening indices, not laboratory values or allowable design stresses.

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Search for the second steel

Current gradeTherma 347HHeat Resistant Steels

Selected grade1.4878 / Therma 321H/4878Heat Resistant Steels

Current-grade chemistry

Second-grade chemistry

Current-grade heat treatment

Second-grade heat treatment

Current-grade applications

Limitations

Second-grade applications

Limitations

Select a second grade to update the comparison.

Sources & Evidence

Sources, methodology and evidence

Sources support grade identity, recorded values or the engineering method. General guidance does not replace product-specific certification.

Grade-specific sources

Outokumpu Therma rangeManufacturer Range Page · Tier A

Reference supporting grade identity or recorded values; interpret it within the cited product and condition context. Role: manufacturer range page.

View source

Outokumpu Therma range datasheetManufacturer data sheet · Tier A

Reference supporting grade identity or recorded values; interpret it within the cited product and condition context. Role: manufacturer datasheet.

View source