Distortion Analysis of Landing Gear during Oil Quench

Problem

Part:

2.5 meter tall landing gear
0.25 meter main tube diameter
AISI 300M Material

Problem:

Large distortions after oil quenching in the following distortion modes:
Bow in XY-Plane
Bow in YZ-Plane
Straightness of a Blind Hole
All distortion modes shown in the figures make assembly of the entire structure very difficult
Immersion into the oil tank is the main focus of the distortion analysis

Process Description

Part is austenitized in a pit furnace at 875° C
Approximately 45 seconds is required to remove the landing gear from the pit furnace
Approximately 75 second open air transfer from pit furnace to oil quench tank
Landing gear is immersed into the oil with a speed of 203.2 mm/sec, with the immersion direction shown in the figure. It takes 11.885 seconds to immerse the entire landing gear in the oil tank.
The landing gear is held in the oil for 5 minutes
Tempering not considered in the model, due to negligible effects on distortion

Model Description

3 surfaces defined for heat transfer boundary conditions
Oil flow stagnation is expected inside the main tube (Inner Surface) and the blind hole.
Different thermal boundary conditions are applied to the outer surface and the inner surface, as shown in the plot below
The blind hole and the inner surface have the same thermal boundary conditions in the baseline model
During immersion, oil enters the blind hole first and then begins to fill up the main tube
In the baseline model, the oil level rising speed inside the bore is assumed to be 20% of the landing gear immersion speed

Model contains 281,265 nodes and 258,272 hex elements

Blind Hole Quench Rate Sensitivity

Heat transfer is increased in the blind hole during the immersion process; all other heat transfer rates remain the same as the baseline model during immersion
All heat transfer rates are identical to the baseline after the part is fully immersed in the oil
Baseline model assumes blind hole heat transfer is equivalent to the main tube inner diameter heat transfer during and after the immersion process
Rate 2 has a faster heat transfer rate than the baseline
Rate 1 has a faster heat transfer rate than Rate 2
The figure shows a significant difference in temperature between the three cases at the end of the immersion process

Heat transfer rates explored in the blind hole do not contribute to the tilting of the blind hole
Figure below shows that the angle of the hole is the same, regardless of the quench rate
Modification of the blind hole to increase the heat transfer rate in the hole to help improve the straightness of the blind hole is not necessary

Heat transfer rates explored in the blind hole do not contribute significantly to the bow distortion in the XY-Plane or the YZ-Plane
Figure above shows that the bow distortion is made slightly worse by increasing the heat transfer rate in the blind hole during immersion, but is not significantly worse
Modification of the blind hole to increase the heat transfer rate in the hole to help improve the bow distortion is not necessary

Structural Arm Quench Rate Sensitivity

Reduced heat transfer of the structural arm is examined
- Oil flow stagnancy is assumed to reduce heat transfer rate on arm
- 2 slower heat transfer rates compared with baseline
- Baseline assumes the same heat transfer rate on the structural arm as on the main tube OD
Figure below shows the reduced heat transfer rate surfaces of the structural arm

Rate 1 is slower than Baseline
Rate 2 is slower than Rate 1
Figure above shows the temperature difference in the structural beam at the end of immersion process
Approximately 100° C difference between Baseline and Rate 1
Approximately 200 ° C difference between Baseline and Rate 2

Bow distortion in xy-plane has a non-linear response to oil stagnancy around the structural beam
Rate 1 produced the least amount of bow in xy-plane
Baseline produces the greatest amount of bow in xy-plane

Distortion of blind hole has a non-linear response to oil stagnancy around the structural beam
Rate 1 produced the straightest blind hole
Baseline produces the greatest amount of distortion of the blind hole

Bow distortion in yz-plane has no sensitivity to oil stagnancy around the structural beam
The non-symmetric mass near the top of the landing gear has the most influence on the yz-plane bow distortion

Figure shows lower bainite phase fraction at the end of the quenching process
- Slower heat transfer rate of the structural beam results in significantly different amounts of lower bainite
The slower the heat transfer, the more lower bainite formed
Increased amounts of bainite reduce bow distortion in xy-plane, but the response is non-linear
- Rate 2 caused slightly more distortion than Rate 1, but less distortion than the Baseline
Increased amounts of bainite reduce distortion of the blind hole, but the response is non-linear
- Rate 2 caused slightly more distortion than Rate 1, but less distortion than the Baseline

Oil Fill Rate in Main Tube Sensitivity

The rate at which the oil fills the main tube is critical to the phase transformation timings and the phases formed
The immersion speed of the landing gear is 203.2 mm/sec
Baseline assumes the inside of the tube fills up at 20% of this value (40.64 mm/sec)
Three different fill speeds were explored
- 50% (101.6 mm/sec)
- 100% (203.2 mm/sec)
- 200% (406.4 mm/sec) Assumes pressure build up forces oil up the inside of the tube
Figure compares temperature inside tube at end of immersion for four cases

The oil fill rate of the main tube during the immersion process has a very significant effect on all three modes of distortion

From top left, moving clockwise:

Bow distortion in yz-plane has a non-linear response to the fill speed
- 50% produces the worst bow
- 100% & 200% are very similar, with 200% slightly worse
Bow distortion in xy-plane has a non-linear response to the fill speed
- 50% produces the least bow
- 100% produces the worst bow
Straightness of the blind hole has a linear response to the fill speed
- Slowest fill speed has least distortion
- Fastest fill speed has the worst distortion

Difference in lower bainite was the cause for differences in distortion with respect to oil stagnancy around the structural beam previously shown
Differences in distortion from the oil fill rate of the main tube are not caused by microstructural phase differences
Figures show that Martensite and Lower Bainite are the same for all fill speeds
Differences in distortion are caused by the transformation timing along the axis of the landing gear

Immersion Direction Sensitivity

Distortion sensitivity to the immersion direction was examined
Figure compares temperature profile at the end of the immersion process for the two immersion directions
The Baseline has oil enter the blind hole first and then fill up the tube at a rate that is 20% of the immersion speed
- Oil spills over the top of the tube and the tube is flooded with oil
The reversed immersion has oil enter the tube first and fills at the immersion speed
Reversing the immersion direction also reverses the axial temperature gradient
- Martensite transformation starts at the open tube end when the immersion direction is reversed
- Martensite transformation starts by the blind hole first for the Baseline
- Reversing the axial phase transformation gradient can have significant effects on bow distortion and axial displacement

Figure below shows the vertical displacement around the blind hole for the Baseline and the Reversed Immersion
Reversing the immersion direction had a very minor impact on the straightness of the blind hole
- Closed side of blind hole was pulled further down by reversing the immersion direction, but the closed side was not pulled up as much

Figure above shows the bow distortion in the XY-Plane for the Baseline and the Reversed Immersion
Reversing the immersion direction has a significant effect on the bow distortion in the XY-Plane, nearly doubling it
Reversing the immersion direction has no effect on the bow distortion in the YZ-Plane

Results

Four process parameters were evaluated for distortion sensitivities for a large landing gear component:
- Oil stagnancy inside a blind hole, oil stagnancy around a structural support beam, oil fill rate into the main tube as the landing gear is lowered into the oil tank, and immersion direction of the landing gear
Three distortion modes were evaluated:
- Bow distortion in XY-Plane, bow distortion in YZ-Plane, and straightness of a blind hole
Bow distortion in the XY-Plane IS significantly affected by oil stagnancy around structural support beam, oil fill rate up the main tube, and the immersion direction
- Bow distortion in the XY-Plane is mainly controlled by the behavior of the structural support beam
Bow distortion in the XY-Plane IS NOT significantly affected by oil stagnancy in the blind hole
Bow distortion in the YZ-Plane IS significantly affected by oil fill rate of the main tube
- Bow distortion in the YZ-Plane is mainly controlled by a fitting near the open end of the tube that contributes to non-symmetric mass around the main tube in that area
Bow distortion in the YZ-Plane IS NOT significantly affected by oil stagnancy in the blind hole, oil stagnancy around the structural support beam, or the immersion direction

Straightness of the blind hole IS significantly affected by oil stagnancy around structural support beam and the oil fill rate up the main tube
- Straightness of the blind hole is mainly controlled by the structural support beam behavior
Straightness of the blind hole IS NOT significantly affected by oil stagnancy inside the blind hole or the immersion direction
Modifications to the quenching process were made to improve the distortion response of the landing gear
- Modeling results were used to direct the modifications
- Customer considered changes proprietary and did not share
Benefit of using heat treatment simulation over physical experiments to perform sensitivity studies was shown
- Ability to modify, and see the effects of, just one process parameter with simulation is easy
- Ability to modify, and see the effects of, just one process parameter with experiments is very difficult, if not impossible
- Cost of simulation is minimal
- Cost of physical experiments can be very high

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