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Engineering Failure Analysis 15 (2008) 835–846

www.elsevier.com/locate/engfailanal

Failure analysis of AISI 304 stainless steel shaft

R.W. Fuller a,*, J.Q. Ehrgott Jr. b, W.F. Heard b, S.D. Robert b, R.D. Stinson b,K. Solanki c, M.F. Horstemeyer c

a Entergy Operations, Design Engineering-Mechanical Systems, Grand Gulf Nuclear Station, Bald Hill Road, Port Gibson,

MS 39150, United Statesb US Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199, United States

c Mississippi State University, Center for Advanced Vehicular Systems, Mississippi State, MS 39762, United States

Received 27 August 2007; accepted 17 November 2007Available online 24 November 2007

Abstract

In this paper, we present a failure analysis methodology of a structural component using a conventional 14-step failureanalysis approach. This failure analysis methodology focused on observation, information gathering, preliminary visualexamination and record keeping, nondestructive testing, mechanical testing, selecting/preservation of fracture surfaces,macroscopic examinations, microscopic examinations, metallography, failure mechanism determination, chemicalanalysis, mechanical failure analysis, testing under simulated service conditions, and final analysis and report. Theapplication of this methodology is demonstrated in the failure analysis of a mixer unit shaft made of AISI 304 stainlesssteel. Using this failure analysis approach, we pinpointed the primary mode of failure and developed a means ofcircumventing this type of failure in the future. The results show that the steel shaft failed due to intergranular stresscracking (sensitization during welding) at the heat affected zones (weld plugs).� 2007 Elsevier Ltd. All rights reserved.

Keywords: Mixer shaft; AISI 304 steel; Intergranular stress cracking; Heat affected zones; Fracture surface

1. Introduction

Failure, in general, is the inability of a component, structure, system, or program to function as intended(usually unexpectedly). From a designer point of view, failure analysis relies heavily on the ability to makeaccurate predictions of the strength and fracture of materials under complex loading conditions. In this study,we will primarily focus on the process of analyzing and understanding ‘‘material” failure. Engineeringproducts and systems can fail for numerous reasons ranging from misuse to poor instructions; however, thiseffort will be focused on failure of the constituent materials. We will examine both experimental and analyticaltechniques for failure analysis and prevention.

1350-6307/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2007.11.001

* Corresponding author.E-mail addresses: rfulle1@entergy.com (R.W. Fuller), jay.q.ehrgott@erdc.usace.army.mil (J.Q. Ehrgott Jr.), william.f.heard@

erdc.usace.army.mil (W.F. Heard), stephen.d.robert@erdc.usace.army.mil (S.D. Robert), ryan.d.stinson@erdc.usace.army.mil (R.D.Stinson), kns3@cavs.msstate.edu (K. Solanki).

836 R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846

In this study, failure analysis of drive-train output shafts was conducted by performing the following con-ventional 14-steps (not necessarily in this order): (1) observation; (2) information gathering; (3) preliminaryvisual examination and record keeping; (4) nondestructive testing; (5) mechanical testing; (6) selecting/preser-vation of fracture surfaces; (7) macroscopic examinations; (8) microscopic examinations; (9) metallography;(10) failure mechanism determination; (11) chemical analysis; (12) mechanical failure analysis; (13) testingunder simulated service conditions; and (14) final analysis and report.

This conventional 14-step failure analysis methodology is applicable to complex loading such as the frac-ture of materials due to overloading or the progressive weakening of a material due to fatigue. It can beapplied to both non-metallic and metallic materials. The authors acknowledge that there may be differencesin nondestructive and chemical analyses techniques used for metallic materials in comparison to the non-metallic, but the conventional 14-step failure analysis methodology should remain the same. These 14-stepsare interchangeable and can be reduced to fewer steps depending on the specifics of a given problem.

Finally, the paper documents the results of initial examinations and detailed investigations. It also discussesthe potential root causes of failure in the shaft and the primary mode of failure. Included with the results of thefailure analysis are recommendations for design improvements of the shaft. If implemented, these recommen-dations should circumvent the possibility of failure of future mixer drive-train output shafts.

2. Observation and background

This case study describes the failure analysis of a fractured drive train shaft from a mixer unit that led tocatastrophic consequences in terms of damage to other equipment, loss of production, and risks to workers’health and safety. Failure can also be less than catastrophic if detected early enough. It may mean a state ofdefectiveness, a lapse in manufacturing quality control or inspection effectiveness, the use of incorrect or belowspecification material, unexpected in-service deterioration, or damage through poor operation or mainte-nance. Even in these cases, the financial consequences can be high in terms of rework, repair, or remediation.

The failed component in this case study was a drive-train output shaft from a 15-hp mixer unit. The entireassembly is illustrated in Figs. 1 and 2. The failed shaft was made of AISI 304 stainless steel (SS) and was onlyin service for three weeks before experiencing these failures, which raised concerns at the manufacturing com-pany. Consequently, root cause analysis aimed to establish the technical facts surrounding the failure by givinga scientific description of the damaged parts, giving an analysis from the standpoint of the materials and theengineering mechanisms by which the damage could have occurred, and finally, suggestions for improvements.

A thorough background investigation was conducted to identify possible contributing factors of failure.Several important facts were uncovered during this process that will be highlighted in detail in the followingsections. The 15-hp mixer motor operates continuously except for changes in voltage. However, the mixer sta-tion is not continuously monitored by an operator. Therefore, no external vibrations were observed prior tothe shaft breaking. Because the shaft was a loose fit, weld plugs were installed using the same AISI 304 SS as

Motor

Center line Belt

M

PCenter line input

Center line output

Center line torque and Gearbox

TG

Center line UHMW Bearing #2

Center line UHMW Bearing #1

Bulkhead Flange

End of Plug Weld (failure section)

mixer

Fig. 1. Side view of 15-hp mixer motor assembly.

Center line input shaft

Center line output shaft

P

T

F

MCenter line of motor

Pulleys

Fig. 2. Motor shaft layout.

R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846 837

the shaft. It is uncertain whether the gearbox was anchored; regardless, the gearbox did not influence shaftfailure. Also, there was no preloading associated with the torque arm that could influence failure. The weightsof the mixer and material in the mixer were not critical and therefore not needed. The temperature of the oper-ational environment is considered to be ambient. The keyway is 12.7-mm square, and the material used for thekey was the same AISI 304 SS as the shaft.

3. Information gathering

The first step of any investigation is to gather data. Information on the technical specification of the equip-ment, the service history, and its mode of operation will be needed for comparison with forensic evidence fromthe failed parts. Such a comparison may indicate a cause of failure. Was the intended material actually used?Were weld sizes according to design? Was the equipment operated correctly? Secondly, the context of the fail-ure must be determined. A site visit to examine and photograph the damaged equipment in place is often help-ful and sometimes essential. It may not be possible to recreate the scene again. What was happening at thetime leading up to the first sign of failure and afterwards? The value of witness statements should not beunderestimated; even when an eyewitness account may not be sufficiently robust for a court of law, it mayprovide valuable pointers to investigators.

4. Preliminary visual examination and record keeping

The primary objective of this investigation was to evaluate the shaft material and describe its failure mode.To fully understand the material and the problem, a detailed background study was performed. This includedan investigation of common failure modes of AISI 304 SS including potential manufacturing problems.

This stage of the investigation was the examination of the damaged parts and failure surfaces (see Fig. 3).This required close attention to detail and meticulous record keeping. Initial examination included photogra-phy, dimensional checks, and possibly preparation of replicas of fracture surfaces. There would be no oppor-tunity to go back later once the component was cut up and pieces removed. Once this on-site examination wascomplete, samples of material were extracted for detailed laboratory examination, chemical analysis, andmechanical testing.

5. Selecting/preservation of fracture surfaces

In this study, we concentrated our efforts on failure information and case studies that related to failures ofoutput shafts. Shafts have a variety of uses and may be subjected to multiple types of loadings from multipledirections. This results in a wide range of potential problems and failure modes that shaft designers must con-sider. Some common failures found in shafts include fatigue as a result of stress concentrations and embrit-tlement. Another common cause for failure in shafts involves misalignment or mismatch of mating parts.This misalignment can cause vibration and ultimately result in a fatigue failure in the shaft [3].

A fractography study of a shaft fracture surface is often used by investigators to identify surface featuresthat indicate the type and origin of failure. Some common surface features found on shaft failures includebeach marks, river marks, striations, ratchet marks, and chevron marks. These features help define the failuresurface. For example, the river marks show the direction of the progression of a crack. They often show up in

Fig. 3. Failed shaft with heat affected zone (HAZ) and keyway.

838 R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846

a relatively fast-growing section of a fatigue zone. Ratchet marks indicate the boundary between two adjacentfailure planes. Ratchet marks along with the size of the final fracture zone help investigators understandwhether loads or stress concentrations were the major cause in a failure [4].

6. Chemical analysis

To determine the case hardening mechanism, if any, spectral analysis was performed on the circumferenceof the shaft using a SPECTRO SpectroMaxx spectrometer. Spectrometer readings were then taken near theshaft center to determine the material composition. The results are shown in Table 1, and the shaft was foundto be AISI 304 stainless steel. This austenitic stainless steel is susceptible to sensitization during welding. Thisoccurs when austenitic stainless steel is heated to temperatures of about 700 �C; at about this temperature, thecarbon content exceeds the solubility limit for austenite. The carbon will bond with the chromium and, given afew minutes, will travel to the grain boundaries as chromium carbide. The grain edges are then depleted ofchromium and thus lose their corrosion resistance. Then, local galvanic corrosion may occur along the grainboundaries. The carbides in the grain boundaries may also cause grain decohesion. To reduce sensitization,the AISI 304L stainless steel may be used because it has less than 0.03% carbon. This shaft could be manu-factured with the ‘‘L” version. The yield strength and ultimate strength for annealed 304 stainless steel are42 ksi and 84 ksi, respectively (39 ksi and 81 ksi for 304L). These strength numbers do not indicate overloadfailure; however, sensitization and subsequent stress corrosion cracking would greatly reduce the shaft’sstrength [2].

Table 1AISI 304 stainless steel composition

Element (%) 304 (1)

Carbon 0.08Chromium 18.00–20.00Manganese 2.00Nickel 8.00–10.50Phosphorus 0.045Sulfur 0.03Silicon 1.00

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7. Mechanical testing

Hardness testing was performed to determine the strength and strength gradient of the shaft. Six readingswere taken starting at the center of the shaft and moving toward the edge. Fig. 4 shows the results of the hard-ness testing as well as the calculated ultimate tensile strength of the shaft at the various locations. The hardnesstesting revealed that the steel was annealed, but the effect of the annealing was lost near the surfaces due to theheat affected zone (HAZ) from the welding.

8. Macroscopic examinations

Samples of the material were extracted from the failed shaft in order to analyze the microstructure. Thesamples were mounted in Epoxy and polished to a mirror finish following a standard preparation procedurefor steels. The specimens were etched with ‘‘Glycergia” (mixture of a nitric acid, hydrochloric acid, and glyc-erol) to reveal the microstructure. The following images were captured with an optical microscope.

Figs. 5 and 6 show that the material had two different phases. The first phase, as shown in Fig. 6, was theaustenitic matrix that could be retained at room temperature by the addition of stabilizing elements such ascarbon, nitrogen, nickel and manganese, which are present in the chemical composition of this material, asshown in Table 1.

0

30

60

90

120

0 0.4 0.8 1.2 1.6Distance from Center of Shaft (in)

Har

dens

s (R

ockw

ell B

)

0

30

60

90

120

Ulti

mat

e Te

nsile

Stre

ngth

(ksi

)

Hardenss (Rockwell B)

Ultimate Tensile Strength (ksi)

Fig. 4. Hardness and ultimate tensile strength as a function of position.

Fig. 5. Microstructure observed with the optical microscope at 20� magnification.

Fig. 6. Microstructure observed with the optical microscope at 50� magnification.

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The grain size of the austenitic matrix varied between 10 and 30 lm. The lamellar structure observed inFig. 7 was presumably chromium carbide precipitates often referred to as Cr23C6 (properly (Cr, Fe)23C6 or(Cr, Fe, Mo)23C6), which could be precipitated during welding and produce intergranular cracking.

The second phase was distributed unevenly along the entire austenitic matrix, as shown in Fig. 8. This phaseis referred to as d-ferrite stringers. This phase improves the resistance (in some environments) of the materialto form microfissures or intergranular cracks adjacent to the weld because they block the propagation ofcracks. However, if the temperature during welding reaches high values, this steel can become sensitizedand lose this property [2].

9. Microscopic examinations

The fractured specimen was examined under a scanning electron microscope (SEM). The fracture surfacewas cleaned, and the specimen was inserted into the SEM. The analysis of the surface began at the far left, asshown in Fig. 9. This location was very close to the edge of the part, and the surface was filled with inter-gran-ular cracks as shown in Fig. 8. These inter-granular cracks were centered about the weld location and quicklydisappeared when moving away from the weld. Near the weld locations, the inter-granular cracks were thefarthest from the surface, going in a distance of approximately 150–200 lm. Farther away from the weld loca-tion, the cracking was less severe but still present, as shown in the bottom right of Fig. 8. An example of sen-sitization in AISI 304 stainless steel was shown in Fig. 7; this compares well with the intergranular cracking inFig. 8.

Fig. 7. An example of sensitization in 304 stainless steel (optical microscopy) [2].

Fig. 9. Location of SEM fractographies as seen on failed specimen.

Fig. 8. Inter-granular cracking near the surface.

R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846 841

While the inter-granular cracking was the most prevalent at the weld locations, chevron marks were locatedthroughout the part. The chevron marks pointed from each of the three weld locations toward the keyway.Figs. 10 and 11 show SEM pictures of the chevron marks. The chevron marks can be seen with the naked eye.

The entire fracture surface, except for one region, had chevron marks covering the surface. The fracturesurface, which formed from a large chunk being removed from beside the keyway, did not have chevronmarks. This area of the fracture surface, which was not on the same plane as the rest, is shown in Fig. 12.

On the fracture surface of the shaft, one section was clearly plastically deformed. This region was observedunder the SEM with Fig. 13 being on the left side of the crack shown in Fig. 12. Fig. 14 was on the right side ofthe crack shown in Fig. 12. Figs. 13 and 14 were taken at the same magnification. The left side displayed brittlefracture characteristics, and the right side displayed ductile fracture characteristics.

The SEM pictures indicate that the shaft failed due to stress corrosion cracking from sensitization. Thecracks began in the HAZ of the weld since this area cooled slowly, allowing time for chromium carbides toprecipitate out to the grain boundaries. This caused inter-granular cracking within the HAZ that quickly

Fig. 10. Chevron marks at shaft surface.

Fig. 11. Chevron marks leading towards center.

Fig. 12. Highly deformed region formed beside the keyway.

842 R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846

Fig. 13. Brittle fracture near the keyway.

Fig. 14. Ductile fracture near the keyway.

R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846 843

propagated from the welds to the keyway. Once the remaining cross-sectional area could no longer hold thetorsional load, the shaft broke into two pieces in a ductile manner. This failure may have occurred only a fewtimes if the mixer builder used stainless steel only for a few mixers, if the other shafts contained less carbon, orif these failed shafts experienced longer welding times.

10. Mechanical failure analysis

A recent study by Sofronas [6] regarding rotating members, where failure was believed to occur fromreverse bending at HAZ, provided steps for analyzing the mechanical failure. A rotating shaft loaded by sta-tionary bending and torsional moments was stressed by reverse bending because of shaft rotation. The tor-sional stress varied with shaft speed. The following procedure was used to determine the safety factor ofthe shaft.

The failed shaft was made of AISI 304 stainless steel. The mechanical properties associated with the failedshaft are listed in Table 2. A schematic/free-body diagram is shown in Fig. 15. The bending forces on the shafthad to account for each of the two belts on the pulley. Forces on the pulleys were found from pulley relation-ships [5]. The forces due to the pulley and torque arm were summed to get the resultant bearing forces actingon the shaft. After solving for the forces of the bearings, pulley, and torque arm, the bending stress was cal-

Table 2Mechanical properties of failed axial shaft and AISI 304

Material Yield strength (kpsi) Ultimate strength (kpsi)

AISI 304 40.0 82.4Failed shaft 42.0 84.0

Bearing #1

Bearing #2

Fy #1

Fx #1

Fy #2

Fx #2

Shaft Torque

Torque Arm

Fy T

Fx T PulleysFx P

Fy P

Gearbox

Fy #1 – Y direction reaction force at bearing #1 Fx #1 – X direction reaction force at bearing #1 Fy #2 – Y direction reaction force at bearing #2 Fx #2 – X direction reaction force at bearing #2 Fy T – Y direction reaction force at torque arm Fx T – X direction reaction force at torque arm Fy P – Y direction reaction force at pulley Fx P – X direction reaction force at pulley

Fig. 15. Free-body diagram of loads applied to shaft.

844 R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846

culated at the failure zone using the force distributions. The shearing stress was calculated from the torsioncreated by the shaft rotation. The stress concentrations at the weld plugs were approximated by assuminga sharp corner fillet. For bending, the stress concentration factor was 4.7. For torsion, the stress concentrationfactor was 1.48. The radius of curvature was assumed to be very small to maximize the stresses. In addition, itwas known that the amperage of the mixer unit varied. This variance in amperage/voltage would cause thespeed of the shaft to vary. Using the given speed vs torque relationship, the torque was determined for 2%speeds. These percentage speeds and percentage torques were put into the pulley force equations and torqueequation for rotation. The average or median speed was assumed to be 100% speed and 100% torque. Table 3lists the variances of speed, the torque multipliers, and safety factors. These amplitude and mean torques andmoments were substituted into the Gerber relation [5] along with the endurance strength and ultimate strengthto determine the safety factor. The safety factor at failure was determined to be 0.66 with speed varying to 98%nominal. The process was repeated for speed varying to 99% nominal; the safety factor was 1.001. A safetyfactor that is less than 1.0 indicates that stresses experienced during actual operating conditions were in excessof the intended allowable design stress. A typical (or ideal) safety factor would be at least 1.50–4 [5].

Table 3Service factor of 1.15 [5]

Percent speed 99 98

Torque multiplier 1.6081 2.1351Safety factor with stress concentrations 1.001 0.66Safety factor without stress concentrations 3.43 2.124Fy#1 (lbf) Y direction reaction force at bearing #1 60.7 55.2Fx#1 (lbf) X direction reaction force at bearing #1 240.6 292.5Fy#2 (lbf) Y direction reaction force at bearing #2 �466.8 �387.1Fx#2 (lbf) X direction reaction force at bearing #2 �2677.0 �3384.0T – torque arm force (lbf) 1445.0 2096.0P – pulley force (lbf) 1159.0 1170.0Torque (in-lbf) 2336 3133.0Alternating bending stresses, ra (ksi) 2.729 5.235Alternating torsional shear stresses, sa (ksi) 2.165 4.087Mean bending stresses, rm (ksi) 9.843 9.843Mean torsional shear stresses, sm (ksi) 3.467 3.467

R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846 845

11. Determine failure mechanism

The primary objective of a material’s failure analysis is to determine the root cause of failure. Whetherdealing with metallic or non-metallic materials, the root cause can normally be assigned to one of four cat-egories; i.e., design, manufacturing, service, or material. Often, several adverse conditions contribute to thefailure. Many of the potential root causes of failure are common to metallic and non-metallic materials. Inthis case study, shafts in general have a variety of uses, and they may be subjected to multiple types of load-ings from multiple directions. This results in a wide range of potential problems and failure modes that shaftdesigners must consider. Some common failures found in shafts include fatigue as a result of stress concen-trations and embrittlement. Another common cause for failure in shafts includes misalignment or mismatchof mating parts. This misalignment can cause vibration and ultimately result in a fatigue failure in the shaft[6].

A fractography study of a shaft fracture surface is often used by investigators to determine fracturemodes and mechanisms of failure. Typically, examination at higher magnification using both light and scan-ning electron microscopes, metallographic sectioning, and chemical analysis will be involved to identify finalfracture modes such as ductile tearing, cleavage fracture, or creep rupture and mechanisms preceding failuresuch as fatigue, stress corrosion cracking, and embrittlement. Micro-analysis in the SEM is often particu-larly revealing. It can reveal defects in welds and the quality of welding and weld microstructure. Chemicalanalysis can confirm whether the materials were as specified and detect corrosion products. Mechanical test-ing for tensile and fracture properties can provide quantitative measures of strength and susceptibility todefects.

Some of the common surface indicators for a shaft failure include beach marks, river marks, striations,ratchet marks, and chevron marks (see Figs. 10 and 11). These features help define the failure surface.For example, the river marks show the direction of the progression of a crack. They often show up in a rel-atively fast growing section of a fatigue zone. Another example is ratchet marks, which indicate the bound-ary between two adjacent failure planes. The ratchet marks, along with the size of the final fracture zone, canhelp investigators understand whether the loads or the stress concentrations were the major cause of a failure[1].

In one particular case study that was investigated, a motor shaft was subjected to rotational bending andfailed due to a fatigue crack that resulted from the dreaded chromium-rich carbide precipitation, consider-able grain growth, and subsequent decohesion of the grains (see Fig. 8). This phenomenon would, of course,be in the heat affected zones of the welds. Welding austenitic steels in large pieces with sections a big as thisshaft precludes any high-temperature re-solutioning, plus the fact that the cracks would still be thereanyway.

12. Final analysis and report

It appears that the failures were aligned with the welds. The weld plugs were used to set the shaft because ofvibration issues due to a loose fit and could potentially fail prematurely. However, the weld plugs were creat-ing stress concentrations that were transporting the failure to the shaft.

Several actions may be taken to prevent this type of failure. If the operational environment allows, the AISI1018 cold-drawn steel could be used because it is better suited for welding. If this is not possible, an AISI 304stainless steel may be used with precautionary measures. The shaft could be heat treated after welding. Byheating the welded joint to 500–800 �C followed by water quenching, the chromium carbides can be dissolvedand returned to solid form. If heat treatment is not an option, the addition of specific alloying elements wouldprevent chromium carbide formation; these elements combine with the carbon in the steel so that chromiumcarbides cannot form. Niobium, tantalum, and titanium are typical alloying elements used to prevent sensiti-zation. These elements have a greater affinity for carbon than chromium. Alloys with these additions are saidto be in a stabilized condition. Another solution for the stainless steel is to lower the carbon content to about0.03% by weight or less so that significant amounts of chromium carbides cannot precipitate. Type 304L stain-less steel has its carbon content at this low level.

846 R.W. Fuller et al. / Engineering Failure Analysis 15 (2008) 835–846

Acknowledgements

We thank the Center for Advanced Vehicular Systems of Mississippi State University for providing mate-rial and testing support and also the Director, Geotechnical and Structures Laboratory, US Army EngineerResearch and Development Center for permission to publish this paper.

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Anal Prevent 2005;5(2):11–5.[5] Shigley, Joseph E, Mischke CR, Budynas R. Mechanical engineering design. 7th ed. Boston: McGraw Hill; 2004 (pp. 323–335, 873–

894, 921–951).[6] Sofronas Anthony. Analytical troubleshooting of process machinery and pressure vessels. Wiley Interscience, A John Wiley & Sons,

Inc., Publication; 2006 (pp. 33, 37, 86, 250, 284).