COMUNICAÇÃO TÉCNICA ______________________________________________________________________________________________________________________________________________________________________________________________________

175735

Occurrance of thermal fatigue cracks and their relation with oxidation Ana Paola Villalva Braga Luiz Gustavo Del Bianchi da Silva Lima

Palestra apresentado no Congresso Anual da ABM, 73., 2018, São

Paulo A série “Comunicação Técnica” compreende trabalhos elaborados por técnicos do IPT, apresentados em eventos, publicados em revistas especializadas ou quando seu conteúdo apresentar relevância pública. ___________________________________________________________________________________________________

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www.ipt.br

OCCURRANCE OF THERMAL FATIGUE CRACKS AND THEIR

RELATION WITH OXIDATION

Ana Paola Villalva Braga, PhD – Laboratory of Metallurgical Processes, Institute

for Technological Research (IPT)

Luiz Gustavo Del Bianchi da Silva Lima, MSc – Laboratory of Surface Phenomena,

Polytechnic School of the University of São Paulo (Poli/USP)

October 4, 2018

INTRODUCTION – Thermal fatigue

Failure by fatigue:

• Formation of a microcrack nucleus;

• Propagation (growth) of the fatigue crack;

• Final rupture.

Thermal fatigue: “A process of repetitive application of thermal cycles, during

which stresses and deformations result exclusively from the imposition of

temperature gradients without the application of mechanical loads [1]”

Restrictions to thermal deformations

Multiple connected cracks: heat checking

[1] BRANDIM, A. S.; SOUSA, R. R. M.; ALVES JR., C. Desenvolvimento de um equipamento

para ensaio de fadiga térmica. Revista Matéria, v. 14, n. 1, p. 749–758, 2009.

𝜎 =𝐸 ∙ 𝛼 ∙ ∆𝑇

1 − 𝜈

2

INTRODUCTION – Failure in tools

• Tool steels will inevitably fail at some point in the mode of damage that develops faster.[2,3]

[2] GONÇALVES, C. S. Efeito do processo de nitretação sob plasma no comportamento em fadiga térmica dos aços ferramenta para moldes

para injeção de alumínio sob pressão. 175 p. Dissertação (Mestrado) — Escola Politécnica da USP, São Paulo, 2012.

[3] BRAGA, A. P. V. Investigação dos mecanismos de nucleação de trincas de fadiga térmica em um aço ferramenta com carbonetos de

nióbio. 210 p. Tese (Doutorado) – Escola Politécnica da USP, São Paulo, 2017.

3

METHODOLOGY

STEEL_1 (H13): matrix

46 HRc

STEEL_2: matrix + NbC

52 HRc STEEL_3: matrix + M7C3 + MC

62 HRc

METHODOLOGY – Hot rolling in pilot scale

5 campaings

5 km rolled

5 campaings

5 km rolled

Hot rolling Hot rolling

AISI 1045 1250°C

AISI 1045 1250°C

5 passes 400 slabs 5 passes 400 slabs

Magnetic particles Magnetic particles

Liquid penetrant

Liquid penetrant

Microscope analysis

Microscope analysis

Cross section analysis

Optical and Scanning

Eletron Microscopy

Thermal fatigue tests [3]

• STEEL_2 only

• Induction heating: 25 kW, 220 kHz

• Internal water cooling: 6.8 l/min

• External calm air cooling

Thermal cycling parameters:

• Maximum temperature (600 and 650°C)

• Minimum temperature (all 110°C)

• Heating speed (1.3 and 2.6 sec)

Characterization

• Surface observation

• Cross section analysis

METHODOLOGY – Thermal fatigue tests

[3] BRAGA, A. P. V. Investigação dos mecanismos de nucleação de

trincas de fadiga térmica em um aço ferramenta com carbonetos de

nióbio. 210 p. Tese (Doutorado) – Escola Politécnica da USP, São

Paulo, 2017.

Thermal and mechanical stresses simulations [4]

ABAQUS/Standard

Surface of hot rolling rolls

Microscopic scale entries from the macroscopic stresses and temperatures simulation

• 1st pass: biggest thermal stresses

• 5th pass: biggest mechanical stresses

• Phase’s properties from literature or macro models

METHODOLOGY – Finite elements modeling

[4] LIMA, L. G. D. B. S. Análise experimental e modelagem numérica da influência da oxidação na fadiga térmica de cilindros de laminação a

quente. Dissertação (mestrado). São Paulo: Escola Politécnica da USP, Departamento de Engenharia Mecânica; 2018.

RESULTS – Rolls’ surfaces: Magnetic particles

RESULTS – Rolls’ surfaces: optical microscopy

Oxidation

marks

Secondary cracks

Main cracks

RESULTS – Rolls’ surfaces: Scanning electron microscopy

Cracks?

No, oxidation!

Oxidation grows through

preferential sites of the

microstructure

RESULTS – Thermal fatigue tests

RESULTS – Mathematical models

100 μm 100 μm

50 μm

Different notch shapes

Application of the history of temperatures calculated on the macroscopic model

Application of the contact pressures of the rolls caused by the slabs on the faces

3 different conditions:

a) Oxidized intact notch

b) Notch without oxidation

c) Oxidized cracked notch

RESULTS – Mathematical models

unnotched

% of the duration of rolling pass

Ac

cu

mu

late

d p

las

tic s

tra

in

Howes[5] discussed the reduction of mechanical properties in the metallic substrate due

to the formation of an oxide layer on the metal surface of the sample.

• Surface oxidation combines the available oxygen with the alloying elements of the

steel, forming oxides of these elements and, at the same time, depriving the metallic

substrate of the positive effect of these constituents on their mechanical properties.

• The ease of diffusion of oxygen by the grain boundaries of the metal, in combination

with the geometric effect of stress concentration of these sites, intensifies the

criticality of the phenomena in these contours, making the metal more susceptible to

failure by the low cycle fatigue mechanism.

Similar analysis[6,7] reported the formation of a network of oxides associated with the

dendrites present in the microstructure of the material.

DISCUSSION

[5] Howes MAH. Study of Thermal Fatigue Mechanisms. In: Spera DA, Mowbray DF. Thermal Fatigue of Materials and Components. Materials

Park, EUA. American Society for Testing and Materials, 1976: 86-105. [6] Oshida Y, Liu HW. Grain Boundary Oxidation and an Analysis of the Effects of Oxidation on Fatigue Crack Nucleation Life. In: Solomon HD,

Halford GR, Kaisand LR, Leis BN. Low Cycle Fatigue. Materials Park, EUA: American Society for Testing and Materials, 1986: 1199-1217. [7] Sonoda A, Hamada S, Noguchi H. Analysis of Small Spalling Mechanism on Hot Rolling Mill Roll Surface. Memoirs of the Faculty of

Engineering, Kyushu University. 2009;69:1-14.

Hochanadel et al.[8]: the continuous oxide layer formed on the substrate is susceptible to

cracking due to the thermal cycle.

Mevrel[9]: Oxide layers generally have lower coefficients of thermal expansion than the

metal substrates on which they grow.

• This difference of expansion induces thermal stresses that, added to the growth

stresses, can cause the cracking and eventual detachment of the oxide.

• The presence of cracks in the oxide generates stress concentrators in the

substrate, facilitating the local low-cycle fatigue process.

• Neu and Sehitoglu[10]: After nucleation of the cracks in the oxide, the regions of the

substrate immediately under the cracks become susceptible again to oxidation, forming

oxide notches in the substrate.

DISCUSSION

[8] Hochanadel PW, Edwards GR, Maguire MC e Baldwin MD. The effect of microstructure on the thermal fatigue resistance of investment cast

and wrought AISI H13 hot work die steel. Proceedings of the 18th International die casting congress and exposition. Indianapolis, EUA, 1995. [9] Mevrel R. Cyclic oxidation of high-temperature alloys. Materials Science and Technology. 1987;3:531-535. [10] Neu RW, Sehitoglu H. Thermomechanical fatigue, oxidation and creep: Part ii. life prediction. Metallurgical Transactions A. 1989;20(A):1769-

1783

DISCUSSION [10]

It is impossible to separate studies of oxidation and thermal fatigue under normal operating conditions of tools exposed to the atmosphere at high temperatures;

In the case where the surface heating temperatures under oxidizing conditions are high enough to allow the oxide formation to exceed the abrasion suffered by the tool, the gradual growth of an oxide layer on the metal surface is expected and has great influence on the nucleation of thermal cracks. Once nucleated, these cracks act as notches that intensify local stresses, leading to propagation of the defects to the metal substrate. Thus, the presence of the oxide layer facilitates the nucleation of defects in the metal surface, by the effect of stress concentration, which favors the mechanism of accumulation of plastic deformation.

Carbides play an important role in the nucleation of secondary and major cracks, as their environment becomes more susceptible to oxidation, before the matrix. In addition, due to its generally polygonal and complex geometry and the difference in coefficient of thermal expansion in relation to the matrix, they act as a stress concentrating notch for crack nucleation, joining the oxidation points in the factors that shorten life in thermal fatigue of the hot forming tools.

CONCLUSIONS

THANK YOU!

Ana Paola Villalva Braga – anapaola@ipt.br

Luiz Gustavo Del Bianchi da Silva Lima – luizlima85@gmail.com