Cause analysis of premature fracture in the welding position of special lifting tool for aero-engines

A specialized lifting tool for aero-engine in a workshop broke down after two months of use, resulting in damage to the workpiece and significant economic losses. The incident also posed a serious production hazard in the workshop. In this study, a series of characterization methods including macroscopic trace observation, fracture analysis, physical and chemical testing, microstructure observation, and hardness testing were conducted to analyze the cause of the lifting tool fracture. Based on simulation methods, the reliability of the lifting tool design was evaluated. The results show that the lifting tool design itself is reliable, and the main reason for the fracture is poor welding quality of the welded structure. It is suspected that there are welding cold cracks in the heat affected zone, which propagate during the use of the lifting tool and eventually lead to fracture. The root cause is that the quality of the base material of the weld did not meet the design requirements, resulting in a higher carbon equivalent in the base material, lower weldability, and increased sensitivity to welding cracks, which was also the reason for the fracture occurring in the welding heat-affected zone. The unqualified material composition affects the process parameters of the pre-welding preheating and the post-welding heat treatment, resulting in the formation of high hardness (724.0HV1) and brittle martensite structure in the welding heat-affected zone.


Introduction
ASTM 4130 steel is a high-strength steel with Mo and Cr as strengthening elements, characterized by high specific strength, hardness, and good toughness [1] .However, due to its high carbon content, it has a tendency for quench hardening and poor weldability, making the welding process complex [2] .The main problems in welding include cold cracks, hot cracks, embrittlement of the heat-affected zone [3][4][5][6] .The strength and toughness of the weld can be ensured by selecting suitable welding materials.However, the heat-affected zone, which has undergone welding heat cycles, has different properties from the base material.In terms of impact toughness, the overheated zone in the heat-affected zone is the weakest part of the joint, and its performance can only be ensured by reasonable welding processes [7]   .Improper selection of welding materials or welding processes can easily cause failure of the welded structure, leading to major accidents and serious economic losses or personal injuries.
This study investigates the failure of an aerospace engine special lifting tool made of AISI 4130 material, which carries a load of approximately 1 ton during operation.The fracture occurred at the welding joints of the cantilever beam, connecting plate, and reinforcement block.The base material hardness before welding is required to be 28-36 HRC, and the welding method is argon arc welding with low hydrogen welding wire.The failed lifting tool was observed and tested for chemical composition, microstructure, fracture morphology, and Vickers hardness using optical microscopy, scanning electron microscopy, and inductively coupled plasma spectrometer.The reliability of the tool design was evaluated through simulation analysis.The factors affecting the strength of the welded structure were analyzed, and the main causes of the tool fracture were identified.The root cause of the fracture failure was further explored to provide a case reference for mechanical equipment failure.

Macroscopic Observation
Figure 1 shows the appearance and fracture surface of the fractured component.The fracture occurred at the location indicated by the arrow in Figure 1(a) during the lifting process.Figure 2(b) shows that there are no obvious serious undercut, porosity, incomplete fusion, weld spatter, or lack of penetration defects in the weld.The weld is straight and symmetrical.The width of the fracture weld is relatively uniform, and the fracture surface is relatively fresh, without obvious signs of original corrosion.The weld structure on the surface of the fractured component is full, indicating that the fracture location is not in the weld zone but likely in the heat-affected zone near the weld.

Metallographic Analysis
Figure 3 shows the microstructure of the welded joint of the fractured component.From the Figure 3(a), it can be seen that the heat-affected zone A is the heat-affected zone near the weld, which is the main distribution area of the cracks.The heat-affected zone B is the heat-affected zone away from the weld.The heat-affected zone A is characterized by coarse quenched grains [8] , mainly consisting of acicular martensite, M-A constituents, and a small amount of bainite.The base material is a tempered sorbite microstructure.

Chemical Composition Analysis
Table 1 shows the chemical composition results of the various structures of the fractured component.
According to the standard ASTM A29/A29M-2020, the elemental content of the material does not meet the requirements for AISI 4130 grade.In particular, the elements C, Mn, and Cr, which affect weldability, hardenability, and brittleness, are exceeded.Carbon equivalent formula (Ceq) and Pcm are estimations of the effect of certain elements on the potential for martensite formation (hardenability) and are presented in Equation ( 1) and (2) [9] .

Hardness Analysis
Table 2 presents the Vickers hardness values of various zones in the welded joint shown in Figure 3.It can be seen from the table that the heat-affected zone A near the weld has higher hardness than the heat-affected zone B and significantly higher hardness than the base material and the weld.

Simulation Analysis
Figure 4 shows the three-dimensional model of the tool components.According to ISO 15614-1-2017, unless otherwise specified before testing, the tensile strength of the test specimen should not be less than the minimum value specified for the corresponding base material.To artificially reduce the reliability of the tool, 80% of the base material strength was taken as the strength of the weld, and the strength properties of the weld and bolt are shown in Table 3.The modulus of all structural components in the model was defined as 210 GPa, and the Poisson's ratio was 0.3.The entire structure was subjected to a self-weight load generated by a 200 kg load.resulting in fusion, incom ficant defect particularly during the u he material, w um-carbon qu o its relativel ening.The w with its car ching 1.04% od to accepta ity to crackin gure 7 [12] .T necessary to tionally, the cessive carbo cold cracks ellent II: ve p between we The fracture primarily occurs in the heat-affected zone near the weld, exhibiting a low ductility intergranular fracture morphology.The microstructure in the heat-affected zone consists of brittle acicular martensite and M-A constituents.The high carbon content in the acicular martensite leads to carbon supersaturation, significant lattice distortion, and poor plasticity, resulting in a hardness of up to 700 HV1, which is much higher than the hardness of the weld and base material.Welding materials with a high carbon equivalent require more stringent welding process parameters.When the process parameters are not properly controlled, the heat-affected zone is prone to cold cracks and embrittlement.The formation of high-carbon martensite is influenced by material composition as well as pre-weld and post-weld heat treatments.The higher the carbon equivalent in the material composition, the higher the hardness of the martensite formed during quenching, leading to increased brittleness and decreased impact toughness [13] .Higher preheating and post-weld heat treatments temperatures and longer dwell times at high temperatures help prevent crack formation.The preheating and post-weld heat treatments temperature is mainly sensitive to the carbon equivalent and is also influenced by the plate thickness.Due to the non-compliant material composition, the actual preheating temperature of the structural components should not follow the theoretical and empirical preheating temperature, as it may result in the formation of brittle structures and welding cracks.Timely post-weld heat treatment is also necessary to eliminate internal stress, temper the quenched structure, improve its toughness, and allow hydrogen to escape from the welded joint, reducing the abnormally high hardness and enhancing the overall performance of the joint [12] .

Conclusion
In conclusion, the possible causes of weld fracture are as follows: (1) The main reason is the poor quality of the weld, with the presence of cold cracks in the heataffected zone, which propagate and eventually lead to fracture during the use of the lifting tool.
(2) The root cause is that the quality of the base material of the weld does not meet the requirements for AISI 4130, with elevated carbon content and excessive levels of Cr and Mn.This results in a high carbon equivalent in the base material, reducing its weldability and increasing the sensitivity to weld cracking.
(3) The indirect cause is the mismatch between the high carbon equivalent and the welding preheating or post-weld heat treatments process, leading to the formation of a high-hardness martensite microstructure in the fracture area of the weld.

1 .
(a) During use; (b) Fracture surface Figure Appearance of the fractured component 2.2.Fracture Analysis The overall weld of the fractured component shows no obvious defects, as shown in Figure 2(a).The outer edge of the fracture surface is predominantly intergranular morphology (Figure 2(b)), while the inner edge is mainly cleavage morphology (Figure 2(c)), with a mixed morphology of intergranular and cleavage in the transition zone.Reinforcement block at low magnification (b) Outer edge of fracture surface; (c) Inner edge of fracture surface Figure 2. Low-magnification and micrographs of the fracture surface

3 .
(a) Welded structure; (b) Microstructure of heat-affected zone A; (c) Microstructure of base material Figure Microstructure of the welded joint of the fractured component Figure 4. T es the stress the bolt reach , as shown weld region and the conn curring in th tress distribu This indicates stem.

Figure 5
Figure 5.dually incre At this point ea between t is still in the e maximum he maximum am, as shown reaches 26.4 and the cant nature, the w olt is still in t hat the safety highly unlike
Fig cussion analysis, the olt experienc rength is ass ng tool far e om obvious tion, the occ n the weld, as lack of f and no signi inal cracks, to fracture d related to th gs to mediu wever, due to quench harde r AISI 4130, I 4130, reac int from goo h susceptibil picted in Fig tment are n cracks.Addit efore, the exc ccurrence of

Table 1 .
Chemical composition of the various structures of the fractured component (Wt.%)

Table 2 .
Vickers hardness values of various zones in the welded joint of the fractured component

Table 3 .
Parameters of the tool components