The nucleation and propagation of a thermal fatigue crack in 4Cr2NiMoV steel

doi:10.1016/S0924-0136(99)00445-8

Journal of Materials Processing Technology

Volume 100, Issues 1–3, 3 April 2000, Pages 63-66

https://doi.org/10.1016/S0924-0136(99)00445-8 Get rights and content

Abstract

From the viewpoint of engineering, the model N_i = k (ΔT − ΔT₀)⁻² of the life of thermal fatigue crack nucleation (TFCN) has been applied in the study of 4Cr2NiMoV steel, and the propagation of a thermal fatigue crack has been investigated for this material. After 910°C quenching and 610°C tempering, 4Cr2NiMoV steel has a better TFCN life and a greater resistance to thermal fatigue crack propagation.

Introduction

A hot-forging die is a kind of tool used for the shaping of red-hot metal. The die bears large compressive stress, local tensile stress and additional bending stress because of the severe impact of the punch. The working surface temperature can reach 300–400°C even 600–800°C at some parts as the die contacts with the red-hot metal (1050–1150°C). After every time of forging, the dies are cooled by spraying water, so the surfaces of the dies undergo frequent shock. Therefore the failure of hot-forging dies is mainly due to wear and thermal cracks, except for the unusual earlier crack of a die block’s swallow-tail. Production practice shows that the thermal fatigue induced by the alternate thermal stress is often the main reason [1] why the hot-forging dies fail in use. Consequently, how to improve the resistance to thermal fatigue of hot-forging dies is always one of the problems of concern to die workers [2].

5CrMnMo and 5CrNiMo are two common hot-forging steels in China. In order to meet the demands for the die materials of hot working, the national standard GB1299-85 had added American steels H13, H11 and the new steel 5Cr4Mo3SiMnVAl and so on, that are being developed within China. 4Cr2NiMoV steel is a new hot-forging die steel. The authors, cooperating with Guiyang steel works, Shijiazhuang tractor factory and Tianjin automobile forging plant, have studied the heat-treatment process, thermal fatigue and the application of 4Cr2NiMoV steel, seeking to improve the properties of the steel [3], [4].

4Cr2NiMoV steel possesses high hot strength and thermal stability, but its thermal fatigue behavior needs to be studied further. In this paper the nucleation and propagation of a thermal fatigue crack for 4Cr2NiMoV are studied in order to provide a criterion for predicting the serve life of the thermal fatigue of 4Cr2NiMoV steel.

Section snippets

Materials and specimen preparation

The chemical composition of test material is listed in Table 1. The size of the specimen is 25 mm × 20 mm × 8 mm. The diameter of the notch root is 0.35 mm, and the notch is cut by φ0.35 mm molybdenum wire. Before testing, the steel specimen is heat-treated with two processes: one is 910°C quenching in oil, and 580°C tempering; whilst the other is 960°C quenching in oil, and 610°C tempering. The structure and property after the heat treatment are listed in Table 2.

Thermal fatigue tests

The surface temperature of the specimen

Data processing

The essence of thermal fatigue is that thermal stress initiates thermal strain so that the thermal strain produces thermal strain fatigue. Thermal stress is expressed as [6] $σ=yαE Δ T (1−2γ)$ where σ is the thermal stress, y the constraint coefficient, α the linear expansion coefficient, E the modulus of elasticity, ΔT the difference of the temperature and γ the Poisson’s ratio. The thermal strain Δε at the notch root induced by the thermal stress is $Δ ε=k_{ε} yα Δ T$ where k_ε is the stress concentration

Conclusions

1.
The thermal fatigue of the hot-forging die steels is caused by thermal stress. The TFCN life can be expressed as N_i = k (ΔT − ΔT₀)⁻². Under the condition of a fixed heat-treatment process, there is a critical temperature difference ΔT₀: when the temperature difference ΔT of the thermal cycles is <ΔT₀, the TFCN life tends to infinity, i.e. thermal fatigue cracks do not nucleate.
2.
4Cr2NiMoV steel quenched at 910°C and tempered at 580°C, ΔT₀ = 350°C; but for quenching at 960°C and tempering at 610°C, its Δ

References (7)

W. Lu et al., The study for increasing the thermal fatigue resistance of HM3 steel, J. Anhui Inst. Technol. 2 (1988)...
F. Xiaozeng et al., The difference of thermal fatigue mechanism and resistance between 3Cr2W8V and 4Cr5MoV1, J. Anhui...
L. Guobin et al., A study of hot-forging die block made of electroslag refining bimetal, J. Hebei Inst. Technol. 3...

There are more references available in the full text version of this article.

Cited by (15)

Crack initiation behavior of a Ni-based SX superalloy under transient thermal stress
2019, Materials Science and Engineering: A
Based on three-dimensional transient thermo-mechanical coupling theory and rate-dependent crystal plasticity theory, a transient thermal shock constitutive model was developed to investigate the crack initiation behavior under transient thermal stress of a V-notched Ni-based single-crystal superalloy at 25 °C ↔ 760 °C/900 °C/1000 °C. The evolution and distribution of the resolved shear stress and damage were simulated. Three surface heat transfer stages were considered during the water-cooling process in the simulation. The crack initiation lives and locations in the simulation results both showed good agreement with those observed in the experiments. The growth directions of the two main cracks were approximately 45° with the dendrite orientation, and the {111}<110> slip family was mainly activated.
Phenomena and degradation mechanisms in the surface layer of die inserts used in the hot forging processes
2017, Engineering Failure Analysis
Citation Excerpt :
An appropriate tool for the physical modeling of this phenomenon is the special test station designed and constructed by the authors, which is based on the spinning disk method, enabling to determine the material's ability to withstand the particular number of thermal changes without the formation of cracks [13]. On the basis of the literature analysis and specialized technical solutions [13,14,15,16,26,27] regarding the testing of the resistance to the operation of high temperature gradients as well as the character of the process of producing a disk-type forging selected for the analysis, a test station was constructed, which represents the temperature cycle of the work of forging dies (Fig. 2). The justification of the selection of this method are thermal fatigue tests performed on samples with a simple geometry, under easily controlled conditions, as well as the possiblity to construct a numerical model representing the experimental test.
The paper presents the results of studies on the analysis of the destructive mechanisms for die inserts used in the first operation of wheel forging process, and the results of laboratory tests obtained on specially constructed research position for testing thermal fatigue and abrasive wear. The results of investigations on the die inserts shown that the dominant destructive mechanisms for these tools are thermal fatigue occurring in the initial the exploitation stage and abrasive wear, which occurs later, and is intensified effects of thermo-mechanical fatigue and oxidation. The observed mechanisms resulting from the extreme conditions prevailing in the hot forging process and are caused by cyclic changes in thermal and mechanical loads. In order to better analysis and knowledge of phenomena associated with these mechanisms, the authors built a special research positions allow for a more complete analysis of each of the mechanisms separately under laboratory conditions, which correspond to the industrial forging processes. A comprehensive analysis of the inserts confirmed by tests on laboratory stands, showed the interaction between the thermal fatigue and abrasive wear, combined with the process of oxidation. The results showed that the thermal fatigue and destruction as a result of oxidation, very often occur together with the mechanism of abrasive wear, creating a synergy effect, causing the acceleration, the most visible and “easily measurable” processes abrasive wear.
Thermal fatigue resistance of H13 steel treated by selective laser surface melting and CrNi alloying
2013, Applied Surface Science
Citation Excerpt :
It is well known that thermal fatigue is resulted by internal stress initiating from cyclic temperature gradient, and it is an important life-limiting factor for many engineering metallic parts, especially for those serving in alternatively heating and cooling conditions such as hot-work dies [1,2]. Accordingly, improving the resistance to thermal fatigue of hot-work die steel is always a subject of major concern to engineering researchers [3]. In previous studies, enhancing the thermal fatigue life of metallic materials was mainly studied by choosing either a right heat treatment conditions or an appropriate chemical composition.
In this study, the selective laser surface melting and laser surface alloying technologies were adopted to improve the thermal fatigue resistance of medium carbon hot-work die steel (H13) by a CO₂ laser. Two kinds of mixed chromium (Cr) and nickel (Ni) powders were used as the laser alloying materials, and the effects of the mixing ratio on the thermal fatigue resistance were investigated thoroughly. Some important results such as cross-sectional morphology, phases, hardness and thermal fatigue behavior were analyzed and evaluated. It indicates that the laser surface alloying technique using mixed powder with ratio of 75%Cr–25%Ni can considerably enhance the thermal fatigue resistance of the H13 steel. The laser alloyed zone has excellent properties such as preventing crack initiation and oxidation corrosion compared with original H13. Thermal cracking and oxidation corrosion that occurred at substrate surface can be surrounded and intercepted by a gridded laser strengthened structure. Therefore, the naturally developed cracks could be effectively prevented. Theses results and analysis show that laser surface technique can be positively used to improve surface mechanical properties of H13 dies.
Properties of 3Cr2W8V Die Steel With Striations Processed by Laser
2009, Journal of Iron and Steel Research International
The striations on the surface of 3Cr2W8V die steel were processed by laser. The microstructure, hardness, wear resistance and thermal fatigue behavior of the specimens processed by laser were measured. The appearance and mechanism of thermal fatigue crack propagation in the zone processed by laser were observed and discussed. The results show that the wear resistance and thermal fatigue resistance of materials processed by laser are all better than those of the unprocessed material. The processed zone by laser plays a role in baffling wearing process and crack propagation. The pile-nail effect of processed zone is the main factor for improving the wear resistance and thermal fatigue resistance of material.
Crack growth behavior of SRR99 single crystal superalloy under thermal fatigue
2008, Rare Metals
The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated. Specimens with V-type notch were tested at the peak temperatures of 900, 1000, and 1100°C. The crack growth curves as a function of the number of cycles were plotted. With the increase of peak temperature, the crack initiation life was shortened dramatically. Through optical microscopy (OM) and scanning electron microscopy (SEM) observation, it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles. The primary cracks generally propagated along a preferential direction. Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.
Thermal fatigue behavior of K465 superalloy
2006, Rare Metals
The thermal fatigue behavior of K465 superalloy was investigated at the peak temperature of 1050°C. By scanning electron microscopy (SEM) and optical microscopy, the main crack length was observed and measured. The initiation sites of the tested alloys are different in as-cast (named as K465) and solution heat treatment (named as SK465) conditions. In K465 alloy, most thermal fatigue cracks nucleate at (Nb, W, Ti)C carbides. In SK465 alloy, thermal fatigue cracks initiate in interdendritic regions, MC-type carbides and some interfaces. Thermal fatigue cracks propagate in transdendritic mode, and M₆C-type carbides could retard thermal fatigue crack growth for SK465 superalloy.

View all citing articles on Scopus

View full text