QT650 vs QT780 vs QT900: choosing the right 1.4313 heat treatment condition
Three tempered conditions of the same martensitic stainless steel — and one decision that governs whether your forging survives its service life. Here is how tempering temperature trades strength against toughness and corrosion, and how to match the condition to the duty.
For X3CrNiMo13-4 (1.4313), QT650 is the toughest and most corrosion-stable condition (700–850 MPa), QT900 is the strongest (900–1100 MPa), and QT780 balances the two (780–980 MPa). The number is a minimum tensile-strength target in MPa, not a temperature. Choose QT900 when the part is stress-limited, QT650 when toughness or long wet-service life governs, and QT780 as the general-purpose default.
Key takeaways
- "QT" means Quenched & Tempered; the number is the minimum tensile strength in MPa, not a furnace temperature.
- All three conditions share one alloy and one hardening step (≈950–1050 °C); only the tempering temperature differs.
- Higher tempering temperature → lower strength, higher toughness. QT650 is tempered hottest and is toughest; QT900 is tempered coolest and is strongest.
- QT650 and QT780 both hold > 70 J impact energy; QT900 drops to > 50 J — the key trade-off line for designers.
- Match to duty: QT900 for turbines/high-speed parts, QT780 for oil & gas/pressure parts, QT650 for hydropower and wet, impact-loaded service.
What does the "QT" number in QT650, QT780 and QT900 actually mean?
The most common misunderstanding about 1.4313 is that QT650 means the part was heated to 650 °C. It does not.
QT stands for Quenched & Tempered, and the three-digit number is the minimum tensile strength level in MPa the finished forging is heat-treated to reach. QT650 corresponds to a lower strength band and QT900 to a much higher one. All three start from the same alloy — a chromium-nickel-molybdenum martensitic stainless steel — and all three are hardened from the same window near 950–1050 °C. What separates them is the tempering temperature applied afterwards.
Tempering is where the metallurgy earns its keep. Freshly quenched martensite is hard and brittle. Reheating it below the transformation point relieves internal stress and refines the structure, trading a little strength for a large gain in toughness and ductility. The higher you temper, the more of that trade you make. This is why the three conditions sit on an inverse curve: the hotter the temper, the softer and tougher the result — and, for this grade, the more stable its corrosion behaviour tends to be.
Higher tempering temperature → lower strength, higher toughness. QT650 is tempered hottest and is the toughest; QT900 is tempered coolest and is the strongest.
QT650, QT780 and QT900 compared: the personality of each
Think of QT650, QT780 and QT900 less as separate grades and more as three temperaments cut from one alloy.
The tough, corrosion-friendly choice. Highest elongation and impact energy of the three. Ideal where the part sits wet for years and impact matters more than peak strength.
The all-rounder. A balanced strength-to-toughness ratio that suits the widest range of duty. The most frequently specified condition for general forged parts.
The high-strength option. Chosen when the design is load- or speed-limited. Elongation and impact fall (A > 12%, KV > 50 J), so it is used where strength headroom is the priority.
1.4313 mechanical properties by condition (comparison table)
These are the property bands specified for X3CrNiMo13-4 (1.4313) forgings under EN 10088-3, confirmed by mechanical testing (ASTM A370) and impact testing (ASTM E23) on the material supplied.
Swipe the table sideways to see all columns →
| Condition | Tensile Rm (MPa) | Yield Re (MPa) | Elongation A | Impact KV | Indicative hardness* |
|---|---|---|---|---|---|
| +QT650 | 700 – 850 | > 520 | > 15 % | > 70 J | ≈ 22–27 HRC |
| +QT780 | 780 – 980 | > 620 | > 15 % | > 70 J | ≈ 26–31 HRC |
| +QT900 | 900 – 1100 | > 800 | > 12 % | > 50 J | ≈ 30–34 HRC |
| +A (annealed) | < 1100 | — | — | — | < 320 HBW |
*Hardness values are indicative for guidance only and are confirmed against the applicable specification on each order. The annealed (+A) row is the soft condition used for machining, not a final delivery condition for load-bearing service.
Notice how little toughness is lost between QT650 and QT780: both hold > 70 J of impact energy while QT780 adds roughly 100 MPa of strength. The real cliff appears at QT900, where impact drops to > 50 J and elongation falls three points. That is the point of diminishing return, where you buy strength with toughness — the single most important line in this table for a designer.
Strength, toughness, corrosion — how do you choose?
Every condition of 1.4313 is corrosion resistant thanks to its ~13% chromium and molybdenum addition, and every condition performs across a wide service window (roughly −60 °C to 300 °C). The condition does not switch corrosion resistance on or off — it fine-tunes the balance between three competing demands.
1 · Strength headroom
If the part is stress-limited — a turbine disc at speed, a valve stem under load, a high-pressure body — you want the yield strength of QT900. More strength lets you keep sections lean without yielding.
2 · Toughness & crack resistance
If the part sees impact, cyclic loading, thick sections or low temperatures, toughness protects against brittle fracture. Here QT650 and QT780 shine, holding their > 70 J impact floor where QT900 gives some back.
3 · Corrosion stability
The softer, higher-tempered microstructure of QT650 is generally the most forgiving in long-immersion, wet or mildly corrosive service. It is the condition we most often recommend where a component stays submerged for years.
Start at QT780 as the default. Move up to QT900 only when the stress analysis demands it, and drop to QT650 when toughness or long wet-service life governs the design.
Which 1.4313 condition for which part?
The clearest way to choose is to see how each condition behaves in the field. These pairings reflect how the grade is typically applied across the hydropower, oil & gas, valve and turbine industries.
Hydropower & wet-service parts
Runner discs, wicket gates and guide vanes that stay immersed. Toughness and stable corrosion behaviour outrank peak strength.
Oil, gas & general pressure parts
Frac pump fluid-end blocks, wellhead spool bodies and pump components — where a balanced strength-toughness ratio holds under high pressure for years.
Turbines & high-speed rotating parts
Gas and steam turbine disks, impellers and seamless rolled guide rings, where high yield strength governs the design at speed.
Marine & heavy corrosion service
Marine propeller shafts, pump and valve parts for saltwater and demanding corrosion environments, where toughness and long-term durability lead. The grade is also specified industry-wide for nuclear pump components.
The heat treatment route behind each condition
All three conditions share the same hardening step and diverge only at the tempering stage. Double tempering is used where the tightest toughness and dimensional stability are required.
Open-die forging near 1150 °C
The billet is worked hot to close porosity and refine grain flow, then air-cooled. This history is common to every condition.
Austenitise 950–1050 °C, then quench
The forging is soaked and cooled in oil or air to transform the structure to martensite — hard, strong and, at this stage, brittle.
Choose the temperature that sets the condition
QT900 tempers at 520–580 °C for maximum strength; QT780 at 550–600 °C for balance; QT650 at 650–700 °C, often double-tempered, for the toughest result.
Test to EN 10088-3 / EN 10250-4
Tensile, impact, hardness, grain size and delta-ferrite checks confirm the target band, and a material test report (MTR) can be supplied with the parts.
How to specify the 1.4313 condition correctly
Because the condition is set in heat treatment and not in forging, the same drawing can be delivered in any of the three states. To avoid ambiguity, put four things on your enquiry:
1. The grade and standard — X3CrNiMo13-4 / 1.4313 to EN 10088-3 (or the matching F6NM / ASTM A182 designation). 2. The target condition — +QT650, +QT780 or +QT900. 3. The governing property — e.g. a minimum yield or minimum impact energy at temperature, so the supplier can confirm it is met in your section thickness. 4. Testing and traceability — third-party witness, MTRs, and any delta-ferrite or grain-size limits.
Section size matters: a very thick forging cools more slowly at its core, so achievable strength at mid-thickness can differ from a thin bar. A capable forging house will tell you honestly whether your target condition is reachable through-section before you commit. If you are unsure which condition your duty calls for, send the drawing and service conditions and we will recommend the condition alongside a quote for your custom 1.4313 forgings.
Frequently asked questions
Does QT650 mean the steel is heated to 650 °C?
No. The number is the minimum tensile strength target in MPa, not a temperature. The condition is reached by hardening near 950–1050 °C and then tempering — which, for QT650, is actually the hottest temper of the three at 650–700 °C.
Which 1.4313 condition has the best corrosion resistance?
QT650 is tempered hottest, giving the softest, toughest microstructure and generally the most stable corrosion behaviour. All conditions share the same base corrosion resistance from chromium and molybdenum; chloride environments still require careful assessment for any condition.
Can one forging be supplied in more than one condition?
Yes. The condition is set during heat treatment, not forging, so the same geometry can be delivered as QT650, QT780 or QT900. You state the target on the order and the property band is verified before shipment.
Is QT900 always the "best" because it's strongest?
No — strongest is not the same as best. QT900 gives up impact toughness (KV > 50 J vs > 70 J) and elongation to reach its strength. For impact-loaded, thick-section or wet-service parts, a lower condition is often more reliable.
What about welding these conditions?
1.4313 has limited weldability due to its low carbon content and should be pre-heated and post-weld heat treated with appropriate procedures and fillers. Weld design is best discussed with the forging supplier before finalising the condition.
Sources & standards
- EN 10088-3:2005 — Stainless steels: technical delivery conditions for semi-finished products, bars, rods and sections of corrosion resisting steels.
- EN 10250-4:2000 — Open die steel forgings for general engineering purposes, Part 4: Stainless steels.
- EN 10222-5:2000 — Steel forgings for pressure purposes, Part 5: Martensitic, austenitic and austenitic-ferritic stainless steels.
- ASTM A182/A182M (F6NM) and ASTM A370 (mechanical testing) — ASTM International.
- Jiangsu Liangyi Co., Ltd. — X3CrNiMo13-4 (1.4313) forgings product data (property bands and heat-treatment windows).
Not sure which condition your part needs?
Send us your drawing and service conditions. We forge X3CrNiMo13-4 (1.4313) in QT650, QT780 and QT900 to EN and ASTM standards, with material traceability and third-party inspection available on request — and we will recommend the right condition before you order.
See our 1.4313 forgings & request a quote → Contact our team