Deformation Distribution and Grain Control in GH4049 Plate Forgings
GH4049 is a nickel-chromium-cobalt precipitation-hardening alloy supplied as forged plate and bar for turbine blades and other hot-section components that must survive long service around 900 °C. It combines high aluminium and titanium content - about 5.1 % to 6.3 % in total - with a narrow forging window, so the metal flows poorly and is sensitive to local coarse grain formation. Of all the defects reported by blade forges, the coarse grain band on the mid-back surface is the most persistent, and the cause is nearly always the same: part of the billet has been worked inside the critical deformation range, where the strain is too small to recrystallise the structure completely.
Grain size is a functional property, not a cosmetic one. A uniform, slightly coarse grain structure is what delivers stress-rupture strength and creep resistance in hot-section parts, while an isolated band of abnormally coarse grains behaves as a weak link that shortens fatigue life and scatters tensile results. Because a blade body is thin and its middle section is restrained by the dies, metal flow during forging is uneven and the mid-region naturally receives very little strain. Splitting the total reduction correctly between the pre-forging and final forging operations is the practical way to keep every part of the forging outside the critical deformation band.
Test Material, Billet Preparation and Heating
The study used GH4049 hot-rolled bar of Φ28 mm with an equiaxed starting structure. Grain size in the as-received condition fell into two populations, approximately 40 μm and approximately 10 μm, each accounting for about half of the measured field - a mixed starting structure that makes the subsequent evolution of grain size easy to track.
Billet size: Φ28 mm × 35 mm, compressed radially with limit pads to control the reduction
Preheating: resistance furnace at 825 °C, holding time 0.8 min/mm of section
Final heating: raised to forging temperature within 10 min, holding time 0.6 min/mm of section
Pre-forging temperature: 1170 °C; final forging temperature: 1175 °C
Equipment: double-disc friction press; upper and lower dies preheated to about 300 °C with pre-heated steel blocks
Surface condition: no lubricant applied; oxide ground off after pre-forging to remove its influence on the second fire
Post-forging heat treatment: 1200 °C × 2 h, air cooled; 1050 °C × 4 h, air cooled; 950 °C × 2 h, air cooled
Grain size measurement: intercept (transversal) method on polished and etched sections
Two-Fire Forging Trial Matrix
Eight groups were forged in two fires so that the pre-forging and final forging reductions could be varied independently. Pre-forging deformation was set at 15 %, 25 % and 35 %, and final forging deformation at 25 %, 35 % and 45 %, giving the matrix below.
| Group | Pre-forging deformation | Final forging deformation |
|---|---|---|
| 1 | 15 % | 25 % |
| 2 | 15 % | 35 % |
| 3 | 15 % | 45 % |
| 4 | 25 % | 25 % |
| 5 | 25 % | 35 % |
| 6 | 25 % | 45 % |
| 7 | 35 % | 25 % |
| 8 | 35 % | 45 % |
Every specimen followed the same cumulative reduction route and the same post-forging heat treatment, so any difference in final grain structure can be attributed to the distribution of strain between the two fires rather than to total reduction or thermal history.
Grain Structure Results
The trials gave a clear and repeatable picture of how the two reductions interact.
When pre-forging deformation is small (15 %), the structure after final forging almost always contains coarse grains together with pronounced grain non-uniformity, whatever final forging reduction is applied. The small first-fire reduction leaves a large part of the billet close to the critical deformation range, and the second fire cannot repair the resulting mixed structure.
Increasing pre-forging deformation to 25 % improves uniformity, but the improvement is not yet reliable at the lower final forging reductions.
A large pre-forging deformation of about 35 % combined with a final forging deformation above 25 % produces a uniform grain structure, and this combination is the only route in the matrix that consistently avoids both coarse grains and banded grain size.
The mechanism is straightforward. The first fire must supply enough strain to drive recrystallisation through the whole section; if it does not, part of the metal remains in the critical deformation band and coarsens during the final fire and the subsequent solution treatment at 1200 °C. A heavy first fire that carries the entire section well past the critical range leaves the second fire with the smaller finishing task it is best suited to, and the final structure remains uniform.
Production Recommendations for GH4049 Blade Forgings
To prevent and eliminate local surface coarse grain on GH4049 blade forgings, the following operating window is recommended.
Set pre-forging deformation at approximately 35 % of total reduction, rather than the 15 % that is still common in shops trying to reduce press load.
Keep final forging deformation above 25 %, and never allow the combined route to leave any region of the forging near the critical deformation range.
Hold the pre-forging temperature of 1170 °C and the final forging temperature of 1175 °C, with holding time calculated from section thickness rather than from a fixed soak.
Remove surface oxide between fires; a rough, oxidised surface changes local friction and flow and worsens strain non-uniformity.
Verify the outcome by intercept grain size measurement on a sectioned forging from each campaign, using the mid-back surface as the reference location, since this is the region most likely to coarsen.
Re-qualify the deformation split whenever billet diameter, die design or press tonnage changes, because all three alter the strain actually delivered to the mid-section.
FAQ
Q: What causes local coarse grain on GH4049 alloy plate forgings?
Working part of the section inside the critical deformation range. The strain is too low to recrystallise the metal completely, and those regions coarsen during the final fire and the following solution treatment.
Q: What pre-forging deformation is recommended?
Approximately 35 % of the total reduction. A small first-fire reduction of about 15 % leaves the structure prone to coarse grains and non-uniformity regardless of the final forging reduction applied later.
Q: How much final forging deformation is needed?
More than 25 %. Below that level, grain uniformity cannot be guaranteed even after a heavy pre-forging reduction.
Q: Why is GH4049 difficult to forge?
The grade is a nickel-chromium-cobalt alloy with high aluminium and titanium content and poor malleability, so it resists deformation and is highly sensitive to its local strain history.
Q: What post-forging heat treatment was used in the trials?
1200 °C for 2 h, air cooled; 1050 °C for 4 h, air cooled; and 950 °C for 2 h, air cooled, applied after the two-fire forging sequence.
Q: How is grain size verified on a finished forging?
By intercept measurement on polished and etched specimens taken from the forging, with particular attention to the mid-back surface where coarse grain is most likely to appear.
