Analise de válvula

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Failure analysis and metallurgical investigation of diesel

engine exhaust valves

Z.W. Yu, X.L. Xu *

Institute of Metal and Technology, Dalian Maritime University, Dalian 116026, PR China

Received 24 September 2004; accepted 16 October 2004Available online 7 April 2005

Abstract

This paper presents a failure analysis of three diesel engine exhaust valves used in a truck which failed in service. The

failure had taken place at the plates of the exhaust valves. The plate material is made from 5Cr21Mn9Ni4N steel. The

fracture surfaces were investigated by SEM and visual observation. The results show that the cracks initiated from

the interface of the tapered plane and the transitional region of the plate and propagated toward the bottom of the valve

for two failed exhaust valves. Fractographic studies indicate fatigue as the dominant mechanism of the failure of the

two exhaust valves. Burning-through occurred on the plate of another failed exhaust valve. The microstructure and

phases of the plates of the failed valves were investigated by SEM and XRD, and compared with those of a new valve.The austenitic matrix decomposed to form the lamellar structure consisting ofc + Cr23C6 at the boundary and within

the grains in service, which is the general metallurgical character of plate material for the three failed valves. The lamel-

lar structure separates the matrix to decrease strength, toughness, the loading ability of the matrix, and gas corrosion

resistance. Formation of the lamellar structure is mainly responsible for the failure of the exhaust valves. The appear-

ance of the lamellar structure in the plate material of the failed valves in service is related to the original metallurgical

structure, and depends on the solution-aging treatment.

2005 Elsevier Ltd. All rights reserved.

Keywords: Exhaust valve; Lamellar structure; Fatigue; Burning

1. Introduction

5Cr21Mn9Ni4N (21-4N) heat-resistant steel [1] is usually used as the plate material of diesel engine ex-

haust valves in the automobile industry, and not only has excellent heat-resistance, but also PbO-corrosion

1350-6307/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2004.10.018

* Corresponding author. Tel.: +86 0411 84729613; fax: +86 0411 84728670.

E-mail address: [email protected] (X.L. Xu).

Engineering Failure Analysis 13 (2006) 673682

www.elsevier.com/locate/engfailanal


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resistance. The 21-4N steel is usually treated by solution-aging in order to strengthen the matrix by a solu-

tion-precipitation strengthening mechanism. The microstructure obtained by suitable solution-aging tech-

nology should be composed of austenite (c) matrix and granular secondary carbides (Cr23C6) dispersivelydistributed (Fig. 1), which precipitate during the aging treatment. Metallurgical examination should be per-

formed on 21-4N steel, to determine the grain size of the austenite and its homogeneity. The amount of the

lamellar structure (Cr23C6 + c) must be determined according to the metallurgical standard [2], which is

specified as less than 15% in the technical demands. However, the matrix without the lamellar structure

at ambient temperature may decompose to form the lamellar structure when in service at high temperature,

and this depends partly on the stability of the matrix at high temperature. The lamellar structure nucleates

at a boundary and grows into the parent matrix, which is known as cellular decomposition and is controlled

by a discontinuous precipitation mechanism [3]. The appearance of the lamellar structure would decrease

the toughness, strength, and loading capacity of the matrix and the gas-corrosion resistance.

The customer returned three failed exhaust valves after a premature failure in service. The failure oc-

curred at the plates of the exhaust valves. The failed exhaust valves were used in a truck with a six cylinderdiesel engine. The main steps involved in the fabrication of the exhaust valve and the related technical de-

mands are given in Table 1.

The paper describes a detailed metallurgical investigation and fractographic analysis on the failed ex-

haust valves. The possible failure causes were assessed.

2. Investigation methods

Specimens were taken from the plates of the failed valves and a new valve, and were prepared by stan-

dard methods for metallurgical examination and XRD tests. The chemical composition of the plate mate-

Fig. 1. Microstructure of plate material after suitable solution-aging treatment.

Table 1

Main fabrication steps and related technical demands of exhaust valve

Materials and heat treatment Rod of valve: 4Cr10Si2Mo Technology:

quenching! tempering

Friction welding! annealing!

electric-plating

Plate of valve: 5Cr12Mn9Ni4N Technology: forging

shaping! solution! aging

Metallurgical examination 5Cr21Mn9Ni4N: Grain grade: 58; amount of lamellar structure: 615%; A small amount of

mixed grain grade

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rial was determined by spectroscopy chemical analysis. The microstructures of the specimen and the frac-

tured surfaces were studied by scanning electron microscopy (SEM) on a Philips XL-30 scanning electron

microscope. The micro-composition of the failed valves was analysed by EDAX. The phases present in the

specimens were determined by X-ray diffraction on a Rigaku D/Max-3A diffractometer with Co Karadiation.

3. Observation results

3.1. Visual observation

The failed exhaust valves are shown in Fig. 2, labeled No. 1, No. 2 and No. 3. The failure had taken

place at the plates of the exhaust valves. The new exhaust valve is labeled No. 4.

3.1.1. Valves No. 1 and No. 2

The fracture section has an angle of 45 to the bottom plane of the valve. The cracks initiated from the

tapered plane of the plate and propagated toward the bottom plane along a direction at 45 to the bottom

plane (Fig. 3). It was noted that there are numerous radial cracks in the plate of valve No. 1 (see arrows).

Beach marks can be observed clearly in the crack propagation zone ( Fig. 4).

3.1.2. Valve No. 3

Serious loss occurred at one side of the plate (Fig. 5). There are two main radial splits in the plate, whose

lengths are, respectively, 15 and 6.5 mm. Both edges of each split were burned through seriously to become

blunt. There are some small circumferential cracks on the tapered plane of both sides of the splits. It was

found that a lot of carbon deposition appears on the plate surface.

Fig. 2. Photograph of failed exhaust valves.

Z.W. Yu, X.L. Xu / Engineering Failure Analysis 13 (2006) 673682 675


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Fig. 3. Macrograph of fractured plates of valves No. 1 and No. 2.

Fig. 4. Macrograph of fractured surfaces of valves No. 1 and No. 2.

Fig. 5. Fractured plate of valve No. 3.



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3.2. Composition examination

The chemical composition of the plate material is shown in Table 2. It can be seen that the composition

of the material is within the specified composition range.Table 2

Chemical composition of plate material (wt.%)

Specimen C N Si Cr Mn Ni Fe

No. 1 0.48 0.42 0.28 22.44 8.61 3.79 Balance

No. 2 0.48 0.41 0.19 21.86 8.96 3.97 Balance

No. 3 0.48 0.37 0.25 22.85 8.55 3.89 Balance

No. 4 0.46 0.45 0.33 22.10 8.68 3.61 Balance

Specified 0.480.58 0.350.50 60.35 20.022.0 8.0010.00 3.504.50 Balance

Fig. 6. SEM image of the fracture surface of valve No. 1.



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3.3. Fractography

3.3.1. Valves No. 1 and No. 2

Low-magnification views of the fracture surface for failed valves No. 1 and No. 2 are shown in Figs. 6and 7, in which the similar fracture characteristics are exhibited. Typical fatigue features such as beach

Fig. 7. SEM image of the fracture surface of valve No. 2.



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marks are exhibited in the propagation zone. There are circumferential and radial cracks on the tapered

plane. The former are attributed to the hot stress resulting from starting-up and shutting-down the engine,

the latter to the mechanical stress resulting from opening and closing the valves. Two fracture planes (frac-

ture A and fracture B) can be observed on the fracture surface. Fracture A and fracture B are produced bytwo independent crack origins. The main crack origin appears at the interface of the tapered plane and the

transition region and toward the bottom plane of the valve. The appearance of beach marks and fatigue

striation (Fig. 8) indicates that both fractures were produced by the propagation of fatigue cracks. Fracture

A was produced before fracture B formed. The micro-crack regions were burned-through to some extent,

which suggests that the cracks formed before fracture A was produced.

3.3.2. Valve No. 3

Serious burning-through appears on the plate surface of valve No. 3. Black coverage can be observed,

whose composition determined by EDAX shows the elements C, O, S, Ca and Na. The boundary and

grains around the main radial cracks were oxidized (Fig. 9). The composition examination in the oxi-

dized region by EDAX shows the elements O, C (and Fe, Cr, Ni attributed to the plate materials of the

valves).

3.4. Metallurgical examination

3.4.1. XRD results

The XRD patterns of specimens No. 1, No. 2, No. 3 and No. 4 are shown in Fig. 10. It can be seen that

all specimens are mainly composed ofc-austenite and Cr23C6. However, compared with the XRD pattern

of specimen No. 4, the diffraction peak intensity of Cr23C6 for specimens No. 1, No. 2, No. 3 increases and

Fig. 8. SEM fractographs of valve No. 1 showing fatigue fracture (a) beach marks on fracture A, (b) fatigue striations on fracture B.



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the diffraction peaks ofr (Fe, Cr) phase appear. It is suggested that the amount of Cr23C6 phase increased

and some r phases precipitated in service.

3.4.2. SEM observation

Fig. 11 is an SEM observation on specimen No. 4 (new exhaust valve). It is composed of a c-austenite

matrix and granular M23C6. However, the grain size of the austenite is larger and serious mixed grains

Fig. 9. Microstructure around the main radial crack (valve No. 3).

Fig. 10. XRD patterns of the plate material of failed valves and new valve.



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appear, compared with the microstructure shown in Fig. 1. The size of the carbide particles is larger and not

well-distributed. There are more carbides in zones with smaller size grain than in those with larger size

grain. No lamellar structure appears in the matrix of specimen No. 4. There are almost no smaller second-

ary carbides appearing in Fig. 1.

Observation on specimens No. 1, No. 2 and No. 3 shows that the matrix decomposed in service at high

temperature to form a lamellar structure consisting ofc + Cr23C6, which is in agreement with the XRD re-

sults. The lamellar structure develops from a grain boundary and grows into the parent grain. Many grainsare filled with lamellar structure (Figs. 12 and 9). The absence of dispersively distributed carbides means

that the austenitic matrix contains more C and Cr to make the austenitic matrix unstable, which results

from the unsuitable solution-aging technology. The unstable matrix would decompose easily to form the

lamellar structure in service at high temperature.

4. Analysis of failure causes

Although the failure modes or fracture modes for valves No. 1, No. 2 and No. 3 have some differences,

the appearance of the lamellar structure in the plate material in service is their general metallurgical char-

Fig. 11. Microstructure of plate material of new valve.

Fig. 12. Microstructure of plate material of fractured plate showing lamellar structure (c + Cr23C6).



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acteristic. The plate-like Cr23C6 and r phases at the boundary or within the grains not only separates the

matrix, but also decreases the toughness, the strength of the matrix and the loading capacity to initiate

cracks at the stress concentration region. The precipitation of Cr23C6 decreases the corrosion-resistance

of the plate material, accelerating gas corrosion along the boundary and within grains (valve No. 3).The original metallurgical structure of the plate material could be also responsible for the failure of the

valves. The inhomogeneous grain size of the austenite matrix is undesirable.

Additionally, the absence of the carbides distributed dispersively at the grain boundary and in the matrix

would weaken the strength at high temperature.

5. Conclusions

1. The failures occurred at the plates of the exhaust valves.

2. For valves No. 1 and No. 2 cracks initiated at the interface of the tapered plane and the transitional

region and propagated toward the bottom plane. Fatigue fracture is their failure mechanism. Burn-

through is the main failure mode for exhaust valve No. 3.

3. The plate material (5Cr21Mn9Ni4N) of the failed valves has similar metallurgical characteristics in that

the austenitic matrix decomposed to form the lamellar structure (c + M23C6) in service. The plate-like

Cr23C6 separates the matrix and decreases the toughness and strength of the matrix so that the cracks

initiated and propagated easily at the stress concentration. The precipitation of Cr23C6 decreases the cor-

rosion resistance of the matrix and results in the burn-through failure of the valve No. 3. The formation

of the lamellar structure is mainly responsible for the failure of the valves.

4. Appearance of the lamellar structure (c + Cr23C6) in plate material in service at high temperature is

related to the original metallurgical characteristics. The unstable matrix structure decomposes to form

a lamellar structure (Cr23C6 + c) in service at high temperature. The unsuitable solution-aging technol-

ogy leads to the unstable metallurgical structure.

References

[1] Weuniann W, et al. Section of materials for internal combustion engine valves. Cleveland (OH): TRW Valve Division; 1986.

[2] Metallurgical examination of the combustion engine intake and exhaust valves, Chinese Mechanical Industry Ministry Standard,

JB/T 6720-93, 1994.

[3] Porter DA, Eastering KE. Phase transformation in metals and alloy. UK: Van Nostrand Reinhold; 1981.


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