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Metallurgical Technologies, Inc.

Analysis Of An Impact Fractured, Welded Steeting Arm Spindle Assembly

By: Metallurgical Technologies, Inc.
Tel: (704) 663-5108
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Spindle Failure Analysis

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SUMMARY:

A welded left front spindle/steering arm assembly that had fractured at the fillet weld joint between the steering arm and inner brake boss was received for analysis to determine the mode of failure. The intact right front spindle/steering arm was submitted for comparison. Results indicate the weld joint failure occurred due to application of a single impact load. Examination of the fractured weld surface revealed a combination of ductile and brittle overload (dimpled rupture and cleavage) fracture, all consistent with impact loading. There are no indications of progressive crack growth via fatigue.

The initiation site for the fracture was a pre-existing crater crack formed during welding. Crater cracks occur at high temperature as the weld metal solidifies at the end of a weld pass. Similar crater cracks were identified at other fillet weld joints on the spindle/steering arm unit as well as in the fillet weld joints of the submitted comparison right front spindle/steering arm unit.

The presence of a dendritic structure with heavy oxidation in the cracks indicates solidification cracking. The presence of nickel plating along the fracture surface of the comparison unit weld crack confirms the cracks were present prior to application of the plating.

Chemical analysis of the steering arm base metal identified it as being SAE 4140 low alloy steel. Microanalysis of the steering arm base metal and the weld filler metal identified the filler metal as being SAE 4140 low alloy steel or a similar grade. An initial weld pass on the comparison unit fillet joint was identified as a grade of austenitic stainless steel. This initial pass using a stainless steel filler was not noted in the weld joint of the failed unit.

Hardness measurements of the cross-sectioned failed weldment identified the weld metal hardness as 27 HRC. The weld heat affected zone (HAZ) hardness ranged from a high of 29 HRC to a low of 19 HRC in the base metal. The spindle was not in a heat treated condition (not austenitized, quenched and tempered), however had been post weld stress relieved.

ANALYSIS:

Two welded steel (left and right front) spindle/steering arm assemblies were received for analysis. The left spindle/steering arm had fractured along the fillet weld joint between the steering arm and inner brake boss. The right front spindle/steering arm was submitted for comparison. The fracture is suspected to be the result of a broadside impact from another vehicle during the xxxxxxxxx Race at xxxxxxxxxx.

Figure 1 is an overall view of the failed assembly in the as-received condition A closer view of the steering arm fracture surface is presented in Figure 2. The fracture is through the complete circumferential joint weld and across the adjacent steering arm. The weld fracture exhibits a pre-existing hot crack (crater crack) further magnified in Figure 3. The crack extends across the weld joint width and is located at the end of the welding pass. The surface of the crack is oxidized with a heavy accumulation of rust. Fracture of the weld joint initiated at the pre-existing crack, and then grew in the directions indicated by arrows in Figure 3.

The other weldments of the spindle and steering arm were also examined. Similar crater cracks were identified in the fillet weld joint of the outer brake boss and support arm (Figures 4 and 5). Examination of weldments of the comparison spindle/steering arm revealed similar crater cracks at the steering arm/inner brake boss fillet weld joint. An example is shown in Figures 6 and 7. (Crater cracks also were observed on the outer brake boss/support arm fillet weld joint.) The cracks are located where the welding process stopped.

The fracture area containing the pre-existing crater crack at the initiation site on the steering arm was cut off and prepared for further analyses. The fracture surface was ultrasonically cleaned with a water base detergent (Alconox) and subjected to magnified examination using a scanning electron microscope (SEM).

Figure 8 is a low magnification SEM photomicrograph of the pre-existing hot/crater crack area and the surrounding fractured weld surface. Different fracture surface sites have been mapped and are shown in detail at higher magnification in subsequent figures. Note the pre-existing crater crack was approximately 0.125 (1/8) inch deep.

Figures 9 and 10 are increasing magnification views of the crater crack fracture surface along the weld outer surface. The higher magnification view of Figure 10 reveals the fracture surface to be severely oxidized indicating it opened during high temperature welding.

Figures 11 and 12 are increasing magnification views of the crater/hot crack surface close to the inner brake boss machined slot (lower hot crack area in Figure 8). The surface exhibits a dendritic structure indicative of a hot solidification crack. A region of ductile overload is also present between this hot crack region and the rest of the crater crack. The ductile overload is the result of a change in fracture planes during the impact fracture.

Figures 13 and 14 are high magnification SEM photomicrographs of the weld fracture surface on each side of the crater crack. Both figures exhibit ductile overload fracture surfaces. No indications of fatigue failure were noted around the crack. Additional examination of the weld fracture surface revealed dimples, which are ductile overload features, confirming failure occurred due to overload and not fatigue.

Figures 15 and 16 are increasing magnifications of the transition region between the weld HAZ and the base metal. The fracture surface topography changes from ductile overload in the HAZ to cleavage in the base metal. The presence of cleavage in the base metal fracture surface is indicative of failure due to high impact loading and a pearlitic (non-quenched and tempered) microstructure.

The SEM examined fracture surface sample was cross-sectioned axially through the crater crack and prepared for metallographic examination in accordance with ASTM E3-01. The polished surface was micro-etched (2% Nital) to reveal the cross-sectioned microstructure. This was done as recommended by ASTM E407-99.

Figure 17 is a montage of two optical photomicrographs exhibiting the complete weldment fracture cross-section. The initiation site and crater/hot crack areas are identified.

Figures 18 and 19 are higher magnifications of the initiation site. Noted is the nickel-plating along the outer weld surface. No nickel plating is observed along the fracture surface suggesting the crater crack was closed at the surface or possibly filled by a thin oxide film during nickel plating.

Figure 20 is a high magnification view of oxidation lining the walls of voids in the weld metal. This also indicates a hot solidification crack.

Figure 21 is a high magnification photomicrograph of the steering arm microstructure away from the HAZ. The structure is primarily pearlite indicating that the base metal originally was in an annealed or normalized condition (not austenitized, quenched and tempered).

The effects of heat input during welding were investigated by microhardness testing across the weldment. A profile was generated of the weld/HAZ/base metal hardness by testing at fine increments across the weldment and into the steering arm base metal. The measurements were made using a Knoop (HK) indenter with a 300-gram load in accordance with ASTM E384-99?1 guidelines. The HK values were converted approximately to Rockwell "C" scale (HRC) using the appropriate formula in ASTM E140-02.

The generated profile is presented in Figure 22. The hardness of the weld metal was 27 HRC at the inner edge of the weld. Hardness of the HAZ metal varied from a high of 29 HRC to a low of 20 HRC. The base metal exhibited a hardness value of 19 HRC. A hardness of 20 HRC corresponds approximately to a tensile strength of 110 ksi. The hardness of the HAZ indicated the weldment was stress relieved and tempered after welding.

Chemical composition of the steering arm body was determined using an optical emission spectrometer (OES) in accordance with ASTM E415-99a. The analysis identified the metal as meeting the requirements of SAE 4140 low alloy steel (Table 1).

Due to sample size restrictions the weld filler metal was analyzed using the energy dispersive x-ray spectrometer (EDS), in conjunction with the SEM. The analysis was conducted in general accordance with ASTM E1508-98 (03). The results indicate the filler metal is a low alloy steel similar to SAE 4140. EDS spectra of the weld filler metal and the steering arm body are presented in Figures 23 and 24 respectively. The displayed spectra are almost identical.

A fillet weld from the comparison spindle/steering arm assembly that also had a crater crack present was axially cross-sectioned through the crack, metallographically prepared and micro-etched. Figure 25 is a montage of the cross-sectioned comparison fillet weld with a crater crack. This crater crack is approximately 0.08 inch deep, somewhat shallower than the 0.125 inch depth in the failed assembly. The initial weld pass (inner pass) does not etch and was made with a stainless steel filler metal. EDS analysis (Figure 26) identified it as a chromium-nickel (austenitic) stainless steel.

Figures 27 and 28 are increasing magnification views of the crater crack surfaces. Figure 28 shows that nickel-plating is present both on the outer weld surface and along both surfaces of the crater crack near the opening. The presence of nickel plating reveals that the crack had been formed and was open prior to the plating process.

Figures 29 and 30 are increasing magnification views of the weld crack tip and adjacent inter-dendritic solidification or "hot" cracks. A layer of oxidation lines the crack surfaces, which is further confirmation of hot - solidification cracking.

CONCLUSIONS:

Fracture of the steering arm/inner brake boss fillet weld was the result of a single impact load. Failure initiated along the weld, at a 1/8-inch deep crater crack that formed on cooling at the end of a weld pass. Similar cracks were noted at other fillet weld joints both on the failed unit and on a comparison unit.

The ductile and cleavage fracture surfaces (with no indications of progressive cracking via fatigue) identify impact overload as the mode of failure. The presence of a dendritic structure with oxidation in the crater crack and inter-dendritic cracks of the failed unit as well as nickel plating along the fracture surface of the comparison unit, confirms hot solidification cracking as the source of the failure initiation.

CHEMICAL ANALYSIS

TABLE I

Chemical Composition (wt. %)

Element
Steering Arm
SAE 4140 Specifications
Carbon
0.39
0.38-0.43
Manganese
0.84
0.75-1.00
Phosphorous
0.013
0.030 max
Sulfur
0.033
0.040 max
Silicon
0.32
0.15-0.35
Chromium
0.87
0.80-1.10
Molybdenum
0.16
0.15-0.25

The steering arm meets the chemical composition requirements of SAE 4140
low alloy steel.

Depth
(mils)
Rockwell
HRC
Knoop
HK
-2
27
290
0
28
294
2
29
304
5
29
307
10
26
287
15
28
296
20
28
296
25
25
280
30
23
265
40
25
280
50
20
249
75
20
253
100
19
247
150
20
253
200
20
253

IMAGES:

Spindle Failure Analysis
Figure 1: An overall view of the left front spindle/steering arm assembly. The steering arm has fractured from the rest of the steering arm body along the fillet weld joint between the steering arm and the inner brake boss. (Photo PA9332)
Spindle Failure Analysis
Figure 2: A closer view of the fractured weld joint on the steering arm. A pre-existing crater crack with a rusted/oxidized surface is visible along the upper portion of the weld fracture surface. The pre-crack is located where the welding process stopped. (Photo PA9336)
fractured weld crater crack
Figure 3: A closer view of the crater crack identified in Figure 2. The crack surface had a heavy rust deposit present prior to ultrasonic cleaning. Weld joint fracture originated from the crater crack. The crack grew in the directions depicted by the arrows. (Photo PA9337)
fractured weld crater crack
Figure 4: A close-up photograph of another crater crack (arrow) in the fillet weld joint of the outer brake boss and support arm. (Photo PA9341)
fractured weld crater crack
Figure 5: A closer view of the crater crack (arrow) identified in Figure 4. The crack is located at the end of a weld pass. (Photos PA9342)
Spindle Failure Analysis
Figure 6: A close-up view of the comparison steering arm and inner brake boss. Another two crater cracks (arrows) are barely visible in the fillet weld joint. A closer view of the cracks is displayed in Figure 7. (Photo PA9339)
hot crack failure
Figure 7: A close-up view of the two crater cracks (arrows) in the steering arm/inner brake boss fillet weld joint identified in Figure 6. Crater cracks were also noted in the outer brake boss/support arm fillet weld joint of the comparison assembly. (Photo PA9340)
hot crack failure
Figure 8: A low magnification SEM photomicrograph of the steering arm weld fracture surface at the pre-existing crater crack following ultrasonic cleaning. The fracture surface has been mapped out. Higher magnification photomicrographs of the various regions are presented in the following figures. (SEM Photo 2S4895, Mag: 14X)
hot crack failure
Figure 9: An increased magnification SEM photomicrograph of the pre-existing crater crack fracture surface along the outer weld surface. A higher magnification view is presented in Figure 10. (SEM Photo 2S4896, Mag: 50X)
hot crack failure
Figure 10: A higher magnification SEM photomicrograph of the pre-existing crater crack fracture surface along the weld outer surface. The fracture surface is heavily oxidized indicating it formed during high temperature welding. (SEM Photo 2S4897, Mag: 500X)
hot crack failure
Figure 11: A SEM photomicrograph of the ductile and hot crack region adjacent to the inner brake boss machined slot. The surface topography is composed of "dimpled" (ductile overload) and dendritic (hot crack) structures. A higher magnification view is presented in Figure 12. (SEM Photo 2S4898, Mag: 50X)
hot crack failure
Figure 12: A higher magnification SEM view of the transition from ductile overload to the dendritic hot crack structure presented in Figure 11. (SEM Photo 2S4900, Mag: 500X)
Spindle Failure Analysis
Figure 13: A high magnification SEM photomicrograph of the ductile overload fracture surface along one side of the pre-existing crack area (upper right region of Figure 8). No indications of fatigue failure were noted between the ductile overload and crater/hot crack. (SEM Photo 2S4903, Mag: 3,000X)
Spindle Failure Analysis
Figure 14: A high magnification SEM photomicrograph of the ductile overload fracture surface along the middle of the weld bead along the other side of the pre-existing hot crack (left side of the crater/hot crack in Figure 8). No indications of fatigue failure were noted between the ductile overload and crater/hot crack. (SEM Photo 2S4906, Mag: 1,000X)
Spindle Failure Analysis
Figure 15:A SEM photomicrograph of the fracture transition from weld HAZ to the base metal. The fracture surface topography changes from ductile overload to brittle cleavage. A higher magnification view is presented in Figure 16. (SEM Photo 2S4910, Mag: 50X)
Spindle Failure Analysis
Figure 16: A higher magnification SEM photomicrograph of the transition from ductile overload fracture (dimples) of the weld HAZ to brittle overload (cleavage) fracture of the base metal. The presence of cleavage indicates failure was due to impact loading and indicates a pearlitic (non-quenched and tempered) microstructure. (SEM Photo 2S4911, Mag: 300X)
Spindle Failure Analysis
Figure 17: A montage of optical photomicrographs of the cross-sectioned failed weld joint at the crater crack and hot crack. (Photos C9258 and C9259, Mag: 15X)
Weld hot cracks, Fillet weld fracture
Figure 18: A higher magnification optical photomicrograph of the crater crack initiation site. A thin nickel-plating layer is observed along the weld outer surface. (Photo C9260, Mag: 100X)
Spindle Failure Analysis
Figure 19:A high magnification optical photomicrograph of the crater crack initiation site. The nickel-plating layer is readily resolved. There is no nickel-plating present along the fracture surface. (Photo C9261 Mag: 500X)
Spindle Failure Analysis
Figure 20:A high magnification optical photomicrograph of oxide-filled voids along the fracture at the hot crack region. (Photo C9262, Mag: 500X)
Spindle Failure Analysis
Figure 21:An optical photomicrograph of the steering arm base metal away from the HAZ. The microstructure is pearlitic, indicating the part is not heat treated (austenitized, quenched and tempered). (Photo C9274, Mag: 100X)
Weld failure & Weld process defects
Figure 22: A hardness traverse profile of the cross-sectioned weld/HAZ/base metal
Spindle Failure Analysis
Figure 23: An EDS spectrum of the steering arm/brake boss fillet weld filler metal. The similarity between this spectrum and the following one of the steering arm indicates a compatible low alloy filler. (SPT6017)
Weld failure & Weld process defects
Figure 24: An EDS spectrum of the steering arm base metal. OES chemical analysis had identified the metal as SAE 4140 low alloy steel. Note the similarity between this spectrum and the weld filler metal in Figure 23. (SPT6018)
hot crack failure
Figure 25: A montage of optical photomicrographs of the cross-sectioned, crater crack in the fillet weld from the comparison spindle outer brake boss/support arm. Note the white stainless steel inner pass. (Photo C9263 and C9264, Mag: 15X)
Spindle Failure Analysis
Figure 26: An EDS spectrum of the inner pass stainless steel weld filler metal identified in Figure 25. The elemental composition indicates a chromium´┐Żnickel (austenitic) stainless steel. (SPT6021)
Weld failure & Weld process defects
Figure 27: A higher magnification optical photomicrograph of the cross-sectioned comparison spindle outer brake boss/support arm cracked weld. A high magnification view of the crack initiation (boxed area) is presented in Figure 28. (Photo C9265, Mag: 50X)
Weld hot cracks, Fillet weld fracture
Figure 28: A high magnification optical photomicrograph of the crack opening presented in Figure 27. Nickel plating lines the fracture surface, indicating the crack was present prior to application of the plating. (Photo C9266, Mag: 200X)
hot crack failure
Figure 29: An optical photomicrograph of inter-dendritic hot cracks lined with oxidation. The presence of oxidation lined inter-dendritic cracks is indicative of hot solidification/crater cracking. (Photo C9269, Mag: 100X)
hot crack failure
Figure 30: A high magnification optical photomicrograph of oxidation in the weld and inter-dendritic cracks. (Photo C9270, Mag: 500X)

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Metallurgical Technologies, Inc. is a Metallurgical Engineering Service Laboratory specializing in Forensic Failure Analysis of Metallurgical Components. They also offer testing, analysis, and consulting services.

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