1.7386

1 What 1.7386 steel is

1.7386 contains about 9 % chromium and 1 % molybdenum. The chromium provides oxidation and scaling resistance in hot steam and flue gas. The molybdenum maintains strength under sustained load at temperature, which gives the steel its creep resistance.

In the annealed condition the structure is ferrite with carbides. After normalising and tempering it is tempered martensite. The two delivery conditions therefore have different strength values, as set out in Table 2.

1.7386 is the plain 9Cr-1Mo grade. It has the same chromium and molybdenum base as 1.4903 (F91) but does not contain the vanadium, niobium and controlled nitrogen additions used in F91.

1.7386 air-hardens. It must be controlled after forging and before welding. Clause 7 sets out the practice we follow.

Designations and synonyms

The same steel appears in specifications and enquiries under all of the following names. They refer to one material, not several.

• EN material number: 1.7386
• EN name: X11CrMo9-1. Older documents use X12CrMo9-1 or X12CrMo9.
• ASTM/ASME forging grade: A182 Grade F9
• UNS number: K90941
• Common shop name: 9Cr-1Mo, or nine chrome

2 Chemical composition of 1.7386

The chemical composition of 1.7386 (X11CrMo9-1) to EN 10253-2 is 0.08 to 0.15 % carbon, 8.0 to 10.0 % chromium and 0.90 to 1.10 % molybdenum, with the balance iron. Full limits are given below in mass percent.

Sulphur is restricted to 0.010 % maximum when the material is ordered to EN 10216-2:2014. Tighter residual limits can be agreed at enquiry stage for hydrogen service or for parts that will be weld overlaid.

3 Mechanical properties

1.7386 is supplied in two delivery conditions. In the isothermally annealed condition (+I) tensile strength is 460 to 640 MPa. After normalising and tempering (+NT) tensile strength is 590 to 740 MPa and the 0.2 % proof strength is almost double. The two conditions are not interchangeable, so state the required condition on the purchase order.

Values apply to test pieces taken and prepared in accordance with the governing product standard. Section size influences achievable properties in heavy forgings. Test location and sampling direction should be agreed before manufacture.

Typical physical properties

Physical properties are typical values for the 9Cr-1Mo family and are given for design guidance only. Use the values in the applicable design code for calculation.

4 International equivalents of 1.7386

The ASTM equivalent of 1.7386 depends on the product form. It is A182 F9 for forgings, flanges and valve parts, A335 P9 for seamless pipe, A213 T9 for boiler tube and A387 Grade 9 for plate. All share the same 9Cr-1Mo base chemistry.

Cross-references are given for identification and enquiry purposes. Equivalent grades are chemically comparable but are not automatically interchangeable. Heat treatment requirements, testing regimes and acceptance criteria differ between standards. Confirm the governing specification with your design authority before substituting one grade for another.

5 1.7386 compared with F91 and F22

1.7386 (F9), 1.4903 (F91) and 1.7380 (F22) are the three grades most often compared in high-temperature piping and pressure work. F22 costs less and is easier to weld. F9 has higher chromium and better oxidation resistance. F91 carries more load at temperature but requires the tightest heat-treatment control.

Substituting F91 for F9 in existing plant is not a direct upgrade. The dissimilar weld between the two grades and the different post-weld heat treatment windows both have to be engineered. Raise this at enquiry stage.

6 Forged 1.7386 products we manufacture

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory. The products below are forged to near-net shape from ingot or continuous-cast bloom in our own shop in Jiangyin, then heat treated, tested and machined to the customer drawing.

Sizes are quoted against the drawing. Send a drawing or a rough envelope, that is outside diameter by inside diameter by height, or length by section, with the required specification and delivery condition. We will confirm feasibility and forged weight.

7 Forging and heat treatment practice

A 9 % chromium steel hardens in still air. A 1.7386 forging left to cool on the shop floor after the last blow forms untempered martensite in the outer skin, and heavy sections can crack before they reach the furnace. Handling after forging is controlled for this reason.

Hot working

• Heat uniformly and soak fully before the first blow. A cold core in a heavy billet is a common cause of internal bursting.
• Work between 1100 °C and 1180 °C and finish above 900 °C. Forging below the finish temperature work-hardens the piece and can cause tearing.
• Apply enough reduction to close the cast structure. A forging ratio of at least 3:1 is normal for pressure parts. Higher ratios are used where through-thickness properties are required.
• Transfer directly to the furnace after forging. Do not air-cool a 1.7386 forging to room temperature before the heat treatment cycle.

Heat treatment

The final cycle is set by the governing specification. On critical work it is approved by the customer before it is run. Two routes are used.

• Annealed (+I). Full or isothermal anneal, furnace cooled. This gives a ferrite and carbide structure with the lower strength values in Table 2. It is used where the part will be heavily machined or extensively welded.
• Normalised and tempered (+NT). Austenitise, cool in air, then temper. This gives tempered martensite and the higher strength and proof values. ASTM A182 F9 requires austenitising and tempering to the minimum temperatures stated in that specification.

Furnace charts are recorded for every cycle and issued with the certificate.

Welding

1.7386 is weldable with matching or 9Cr filler when the thermal cycle is controlled. Typical practice is preheat at 200 to 300 °C, interpass temperature maintained throughout, and post-weld heat treatment at 730 to 760 °C. The weld should cool below the martensite finish temperature before PWHT so that the weld metal transforms fully. Take actual parameters from a qualified WPS for the joint concerned.

Machining

Machine in the annealed condition where the sequence allows. The normalised and tempered condition requires lower speeds, rigid setups and sharp positive-rake tooling. On parts where distortion matters we rough machine before final heat treatment and finish machine afterwards.

8 Testing, inspection and certification

Every 1.7386 forging is supplied with an inspection certificate to EN 10204 3.1. Third-party witnessed inspection to EN 10204 3.2 is available and is arranged before manufacture starts.

• Chemical analysis. Ladle analysis, and product analysis on request.
• Mechanical testing. Tensile, impact and hardness from integral or sacrificial test material, at locations agreed on the drawing.
• Ultrasonic testing. To EN 10228-3 or ASTM A388, to the class stated on the order.
• Magnetic particle or dye penetrant. Surface examination to ASTM A275 or equivalent.
• Positive material identification. PMI on finished parts to confirm the 9Cr chemistry. F9 and F91 cannot be told apart visually.
• Dimensional report. Against the customer drawing, issued with the documentation pack.

Marking, heat traceability and documentation format follow the purchase order. State any pressure equipment directive or project quality plan at enquiry stage so that it is included in the manufacturing plan from the start.

9 Where 1.7386 is used

1.7386 is used for components that hold pressure at elevated temperature over a long service life. Typical applications are listed below.

• Power generation. Superheater and reheater headers, steam collectors, main steam pipework, turbine casings and turbine components.
• Petrochemical and refining. Hydrocracker and hydrotreater internals, catalytic reformer piping, and furnace and heater components in hot hydrogen service.
• Pressure vessels and heat exchangers. Tubesheets, channel covers, girth flanges and shell-and-tube exchanger components on the hot side.
• Valves and fittings. Forged valve bodies, bonnets and closures for high-temperature steam and process lines.

10 Frequently asked questions about 1.7386

11 About the manufacturer

12 Related grades

Other alloy steel grades we forge, with full datasheets.

• 42CrMo4
• 1.7225
• AISI 4140
• 34CrNiMo6
• AISI 4340
• 1.6587
• 18CrNiMo7-6
• AISI 8620
• 17-4PH
• Inconel 625
• Incoloy 800H
• All alloy steel grades