High-cycle fatigue life of coated low-carbon steel

High-cycle fatigue life of coated low-carbon steel A.M. Hashem and I.H. Aly

Faculty of Engineering and Technology, Minia University, Minia, Egypt (Received 19 August 1993; revised 18 October 1993) Reversed bending fatigue tests were carried out to compare the fatigue behaviour of specimens of low-carbon steel with and without coatings of titanium alloy. The tests were performed under completely reversed stress in the high-cycle fatigue range (>103). The results obtained for each test series are subjected to a standard statistical analysis and S - N curves are determined and plotted. Stresr level, fatigue life and probability of failure diagrams are constructed. In this work, the effect of coating thickness (resulting from treatment at different temperature levels) on fatigue behaviour is investigated. The results of this investigation will highlight the influence of the stress level, fatigue life and probability of failure on the coating conditions considered useful in protective coating processes. (Keywords: titanium alloy coating; diffusion coating, low-carbon steel; fatigue life; probability of failure; boundary layer)

Engineers and designers engaged in the development of machines subjected to varying loads are often faced with fatigue problems. Through the years, several methods have been used for handling these problems to arrive at appropriate solutions involving materials, sizes and shape of parts and manufacturing processes 1. Many tests have been conducted to determine the fatigue properties of materials and specific parts. Several volumes would be required to record all the fatigue data collected on parts or test specimens with various forming processes, machining, surface finish, heat treatments, surface environments, chemistries, hardnesses, microstructures and frequency of loading 1-5. While considerable effort has been devoted to

understanding the chemistry of coating and corrosion, little time has been spent on establishing a base for understanding and modelling the deformation of coating and components. This reflects the generally successful use of coatings in many industrial applications 6. Titanium alloys are emerging from the development and experimental application stage. It appears that titanium alloys will find widespread application. Titanium coatings appear to: 1. improve the temperature stability of materialsT,S; 2. have superior corrosion and oxidation resistance propertiesS; and

Table 2 Mechanicalproperties of low-carbon steel

Table 1 Chemicalcomposition of low-carbon steel, wt%

C 0.200

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Figure 1 Geometryand dimensions of the fatigue test specimens (dimensions in mm) 0142-1123/94/050321-06 <~) 1994 Butterworth-HeinemannLtd

Fatigue, 1994, Vol 16, July

321

High-cycle fatigue of coated low-carbon steel: A.M. Hashem and I.H. AIy

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approaches are inadequate. Rather than trying to rationalize coating treated behaviour through a consideration of just fatigue behaviour, the data can then be used to analyse the mechanical behaviour of titanium coatings, as for an uncoated metal component5. These different approaches are illustrated in this paper by reference to the behaviour of uncoated specimens, and coated specimens subjected to three different treatment regimes, under various cyclic stress levels. The material used is low-carbon steel. A statistical treatment is performed giving probability lines for each case 9. The shortcomings of such an approach are highlighted by the fatigue results.

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3. offer excellent applications in atmospheric, chemical and salt water service 8. These coatings have such wide application possibilities that it is becoming apparent that such simple 322

Fatigue, 1994, Vol 16, J u l y

The experiments on diffusion coating of the steel specimens were performed using titanium powder as follows. The specimens were packed in material consisting of 49% ferrotitanium powder (30% Ti) with particle size <63 ~m and 49% of fine alumina used as a dilutent to prevent striking hard. The remaining 2% was ammonium chloride, added to the packed mixture to form the active gaseous phase. The packed boxes were closed and placed in the heating furnace.lO,1l

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The fatigue test specimens, coated and uncoated, were manufactured from low-carbon steel. The chemical composition and mechanical properties of this alloy are listed in Tables 1 and 2 respectively. The asreceived rods of 12.5 mm diameter were machined into cylindrical specimens with a gauge diameter of 4.0 mm and a nominal gauge length of 18 mm. The geometry and dimensions of the fatigue test specimens are shown in Figure 1. After machining, the titanium coatings of the specimens were applied according to three different treatment conditions:

Completely reversed fatigue tests were performed for both the coated and uncoated specimens at three stress levels: ---350 MPa, ___300MPa and -+250 MPa. These stress levels were selected to have a fatigue life greater than 103 cycles, which is considered to be within the high-cycle fatigue range. The fatigue tests were carried out on an Avery fatigue testing machine running at 1420 per min. The fatigue life for each specimen, coated and uncoated, was recorded. All the fatigue specimens were run to fracture. RESULTS AND DISCUSSION

Statistical treatment Statistical treatment of the results was felt to be essential because of the wide scatter of the fatigue test results. The most accepted method to assign the probability level (Pi), especially when the number of tests is relatively small4'9, is

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where i is the order number of the tested specimen, and Z is the total number of specimens tested. According to Equation (1) the corresponding probability of failure (Pi) for each specimen is obtained from the order number of the tested specimen at each stress level. In the present work the value of Z is equal to 9. The values of Pi are therefore 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90%. Using log-normal probability papers and fatigue life, probability lines for all stress levels can be obtained. These values will help in the construction of tr/N/P diagrams.

Probability of failure The fatigue cycle lives N for the coated and uncoated specimens were recorded, then statistically analysed as a function of the probability of failure and plotted on log-probability paper for every stress level. Linear regression analysis was performed and the best-fitting line was determined using the least-square method. Figure 2 represents the probability of failure, Pi%, as a function of the cycles to failure N at different stress levels (350, 300 and 250 MPa) and for various coating conditions (C1, C2 and C3) compared with the uncoated specimens. By making use of the probability lines shown in this figure, the fatigue lives were obtained at four levels

Fatigue, 1994, Vol 16, July 323

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of probability of failure (1%, 10%, 50% and 90%). These are the most commonly accepted levels for design applications 9. The relationship of tr, N and P at these levels is shown in Figure 3.

Fatigue life factor The fatigue life factor (FLF) is a measure of the improvement in fatigue behaviour according to the coating condition. It is defined as the ratio of the fatigue life of a coated specimen to that of the uncoated specimen at the same level of stress, or, and probability of failure level, Pi%: N'

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(2)

where N' is the fatigue life of the coated specimen

324

Fatigue, 1994, Vol 16, July

and N is the fatigue life of uncoated specimen (both in cycles). For various coatings and different stress amplitudes, FLF is less than 1. This shows that the fatigue life of the coated specimens is lower than that of the uncoated specimens. The decrease is caused mainly by the poor adherence between the coating and the core 11-13. That is fairly obvious, because the interaction layer surrounding the core is brittle.

Stress level The effect of stress level of FLF values was studied at four levels of probability for the three different coating conditions (C1, C2 and C3) (Figure 4). It is clear that the optimum coating condition for each stress level is C2 (T=1050°C and t = 6 h ; if the

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The FLF values were compared for the various coating conditions at three stress levels used, and analysed for the four probabilities of failure used (Figure 5). Once again, the conclusion is that the m a x i m u m improvement was gained when coating condition C2 was used. The mechanism of the titanium diffusion coating is shown in Figure 6, which shows the core (low-carbon

Fatigue, 1994, Vol 16, J u l y

325

High-cycle fatigue of coated low-carbon steel: A.M. Hashem and I.H. Aly steel), the boundary layer and the intermediate layer of titanium carbide (TIC). The carbon content of the boundary layer can be different to react with Ti. Therefore the formation of TiC starts as well as a ferrite phase up to 1050°C, when the TiC layer is completely formed. It was found that the fatigue properties of coated specimens are improved at this temperature 12. When the specimen was treated at 1150°C, it was observed that the fatigue life was decreased. This can be attributed to the formation of the T i - H compound on the treated layer 13. However, for this study, the optimum fatigue properties of the titanium diffusion coating were achieved with a coating t e m p e r a t u r e level of 1050°C.

3 4 5 6

7 8

9 10 11

CONCLUSIONS This paper has considered the effect of treatment temperature on the fatigue properties of titaniumcoated steel specimens at the same probability of failure level. The main conclusions are as follows. 1. At the same stress amplitude, the fatigue lives of coated specimens are lower than those of uncoated specimens. 2. The maximum improvement has been gained for the three different coating (C1, C2 and C3) at the various stress levels i.e., at o r = 3 0 0 M P a , FLF = 0.337 for C1, FLF = 0.813 for C2 and FLF = 0.450 for C3 at the same probability (50%). 3. The coating condition of 1050°C at 6 h (C2) leads to the optimum fatigue properties because the most carbon is diffused from the boundary layer to form titanium carbide. The fatigue life is lowered by a treatment temperature of 1150°C; this may be due to the formation of the T i - H compound. REFERENCES 1 Graham, J.A. 'Fatigue Design Handbook', Vol. 4, SAE Inc., 1968, pp. 11-17 2 Forrest, P.G. 'Fatigue of Metals', Pergamon Press, London, 1962, pp. 93-127

326 Fatigue, 1994, Vol 16, July

12 13

Wood, M.I. and Restall, J.E. In 1986, pp. 1215-1226 EI-Hadidy, A.F. Bulletin Faculty of Eng. and Tech., Minia University, Part 1, 11 1992, 109 Shigley, J.E. and Mitchell, L.D. 'Mechanical Engineering Design', McGraw-Hill Inc., 1983, pp. 270-275 Wu, Z., Wang, B., Jin, D., Xie, M., Liu, X. and Lu, S. In 'Proc. Fourth Int. Conf. on Mechanical Behaviour of Materials', Stockholm, Sweden, 15-19 August 1993, pp. 393-400 Ahlroth, R. and Kettunen, P. In 'Proc. Fourth Int. Conf. on Mechanical Behaviour of Materials', Stockholm, Sweden, 15-19 August 1993, pp. 401-407 Miner, D.F. and Seastone, J.B., 'Handbook of Engineering Materials', Part 2, John Wiley & Sons, 1955, pp. 431-433 Horger, O.J. 'Metals Engineering Design', McGraw-Hill, 1965, pp. 242-248 Eckstein, H.J. 'Technologie der Waermebehandung von Stahl', VEB Deutsche Verlag, 1976, pp. 159-170 McGannon, H.E. 'The Making, Shaping and Treating of Steel', USS, New York, 1964, pp. 931-945 Mudrova, A.G. 'Protective Layer in Metals', Vol. 4, Naukova Duma, Kiev, 1971, pp. 172-175 (in Russian) Hirai, S. and Shigetomo, U. Nippon Kinzoku Gakkaishi 1987, 51 (11), 1030

NOMENCLATURE C1, C2 and C3 Coating treatment conditions 1, 2 and 3 respectively E Modulus of elasticity (MPa) FLF Fatigue life factor = N ' / N Fatigue life of uncoated specimen N (cycles) Nt Fatigue life of coated specimen (cycles) Probability of failure level ( % ) ei Percentage reduction of area ( % ) q Temperature T Time (h) t Percentage elongation ( % ) O" Stress level (MPa) o-~ Endurance strength (MPa) Rupture strength (MPa) O'r Ultimate strength (MPa) O'u Yield strength (MPa) O'y