UGI 4418 QT900 Turned Bar

Due to its high percentage of nickel, chromium and molybdenum and its low carbon content, the UGI4418 grade has good atmospheric corrosion resistance. The marine atmosphere resistance of this grade is equivalent to that of 1.4542.

Pitting corrosion: The pitting potentials were measured in a solution containing 0.02M NaCl at 23°C. The results show that the pitting corrosion resistance of the UGI 4418 grade is equivalent to that of grades 1.4542 and 1.4307 (containing 0.02% of sulphur). UGI 4418 also has far better pitting corrosion resistance than a 13% Cr martensitic grade (1.4006).

Intergranular corrosion: Its structure and composition make the UGI 4418 grade insensitive to intergranular corrosion.

Stress corrosion: The UGI 4418 grade resists to stress corrosion cracking more than 720 hours at ambient temperature, in a solution of pH = 4.5 saturated with 10% H₂S*.

* Test conditions to NACE TM0177 for type 630 grades (1.4542).

UGI 4418 can be machined in annealed condition or quenched and tempered condition. Machining UGI 4418 closely depends on the mechanical properties and is therefore similar to that used for structural steels. The machinability of UGI 4418 is similar to that of martensitic grades containing nickel type 1.4057.

Annealing: Softening to about 600°C (do not exceed 625°C)

Quenching: Oil quenching (or air quenching depending on the cross-sections) from 950°C to 1050°C

Tempering: Adjust the temperature according to the level of the mechanical properties required. QT 900 = 590°C to 620°C for 2 x 4 hours or 1 x 8 hours minimum. To avoid the risk of quenching cracks, tempering must be carried out as soon as possible after the quenching operation.

Forging:

Heating between 1150°C and 1180°C is recommended. Hot transformation (forging) must preferably be carried out at a temperature of between 1180°C and 900°C, followed by air cooling. The behaviour of the UGI 4418 grade during forging is equivalent to that of a type 1.4301 austenitic grade. Full heat treatment, austenitization followed by tempering, is recommended after hot transformation.

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UGI 4418 can be welded by electric arc (GTAW, SMAW, etc.) and by most other processes (spot welding, seam welding, LASER, etc.). Due to its low percentage of carbon and to its nickel content, UGI 4418 is easier to weld than most martensitic stainless steels.

The low-carbon martensite combined with the finely dispersed residual austenite gives the UGI4418 HAZ excellent toughness in the as-welded condition. The UGI4418 welds are therefore only slightly susceptible to cold cracking and it is not normally necessary to preheat the parts, unless they are very thick or unless their geometry may generate high concentrations of stress at the welds after cooling (in this case, preheating at 100-120°C is recommended). If the weld metal does not require the mechanical properties of UGI 4418, an austenitic grade such as A316LM or A316LT can be used as filler metal. Post-weld heat treatment is then generally not necessary. If the molten zone must have the same mechanical properties as those of UGI 4418, a homogeneous filler metal can be used. In this case, post-weld heat treatment at 580-600°C is recommended.

MIG welding: For MIG welding with homogeneous filler metal, a slightly oxidizing shielding gas such as Ar + 1-2% CO₂ will be chosen to avoid too high a percentage of oxygen in the weld metal, thus ensuring good impact properties for the weld metal.