Improvement of Fatigue Performances of AISI 4340 Steel Thread Root by Rolling Processes

doi:10.3969/j.issn.1004-132X.2025.10.010

Abstract

Abstract:

The rolling process was adopted to improve the fatigue performances of AISI 4340 steel bolt threads， and the optimal rolling parameters were determined by orthogonal testing with the bolt fatigue life as the evaluation index. The surface integrity and fatigue life of unrolled， under-rolled， optimally rolled， and over-rolled thread were analyzed to elucidate the thread fatigue mechanism. The results show that the rolling parameters have a significant effect on the fatigue life of AISI 4340 steel threads， with the order of significance as follows： rolling depth， spindle speed， rolling times. The optimal parameters are as： rolling depth 0.09 mm， spindle speed 40 r/min， and 3 rolling times. The surface integrity improvement of under-rolled samples is low， and the over-rolled samples have the largest degree of work-hardening but damaged the thread root surface. The surface roughness Sa and Sq of thread root with the optimally rolled are reduced to 0.124 μm and 0.165 μm respectively， and the residual compressive stress is increased to $-$ 247.1 MPa. In addition， transition hardening is avoided with an increase in micro-hardness and the formation of a deeper deformation layer. Due to the synergistic improvement of surface roughness， micro-hardness and residual stress of the threaded root with the optimal rolling parameters， the fatigue strength was increased by 50% compared to unrolled bolts. The results indicate that the rolling process with reasonable parameters achieves the purpose of high-quality strengthening and may effectively solve the problems of high-fatigue fracture of bolts.

Key words: AISI 4340 steel, bolt thread, rolling process, fatigue performance

摘要：

采用滚压加工提高AISI 4340钢螺栓螺纹疲劳性能，以螺栓疲劳寿命为评价指标，通过正交试验优选滚压参数，并分析未滚压、欠滚压、优滚压和过滚压时螺纹的表面完整性与疲劳寿命，以阐明螺纹疲劳机制。结果表明，滚压参数对AISI 4340钢螺纹疲劳寿命有显著影响，显著性顺序为：滚压深度、主轴转速、滚压次数；优选参数为：滚压深度0.09 mm，主轴转速40 r/min，滚压次数3；欠滚压样件表面完整性改善效果低，过滚压样件加工硬化程度最大但损伤螺纹根部表面，优滚压螺纹表面粗糙度Sa和Sq分别降至0.124 μm和0.165 μm，残余压应力增至 $- 247.1 M P a$ ，且显微硬度提高并形成较深梯度变形层，同时避免了过渡硬化；优滚压螺纹根部表面粗糙度、显微硬度和残余应力得到协同改善，比未滚压螺栓的疲劳极限提高了50%。研究结果表明，合理参数的滚压工艺实现了高质强化目的，可有效解决螺栓高周疲劳断裂问题。

关键词: AISI 4340钢, 螺栓螺纹, 滚压加工, 疲劳性能

CLC Number:

TG668

Hongwei LIU, Renke KANG, Xianglong ZHU, Haotian MU, Fangzhou HE, Mo CHEN. Improvement of Fatigue Performances of AISI 4340 Steel Thread Root by Rolling Processes[J]. China Mechanical Engineering, 2025, 36(10): 2241-2248.

刘宏伟, 康仁科, 朱祥龙, 慕昊天, 何方舟, 陈莫. 滚压加工AISI 4340钢螺纹根部疲劳性能的提高[J]. 中国机械工程, 2025, 36(10): 2241-2248.

Figures/Tables 15

Tab.1 Chemical composition of AISI 4340 steel（mass fraction）

w（C）	w（Mn）	w（Si）	w（Cr）	w（Ni）
0.420	0.670	0.300	0.800	1.720
w（Mo）	w（S）	w（P）	w（Fe）
0.250	0.004	0.010	余量

Fig.1 AISI 4340 steel bolt sample

Fig.2 Thread root rolling experiment system

Tab.2 Orthogonal test factors and levels

水平	因素
水平	滚压深度/mm	主轴转速/（r·min^-1）	滚压次数
1	0.05	20	1
2	0.09	40	2
3	0.13	60	3

Tab.3 Experimental scheme and test results

样件	滚压深度/mm	主轴转速/ （r·min^-1）	滚压次数	疲劳寿命（cycle）
1	0.05	20	1	114 976
2	0.05	40	2	145 709
3	0.05	60	3	200 104
4	0.09	20	2	884 302
5	0.09	40	3	1 015 503
6	0.09	60	1	642 603
7	0.13	20	3	290 578
8	0.13	40	1	483 178
9	0.13	60	2	383 164
10	车削对照组			76 322

Tab.4 Fatigue life range analysis

指标		滚压深度	主轴转速	滚压次数
疲劳寿命（cycle）	k₁	153 596	429 952	413 585
	k₂	847 469	548 130	471 058
	k₃	385 640	408 623	502 061
	R_j	693 873	139 566	88 476

Fig.3 Analysis of rolling parameters on fatigue life

Fig.4 Surface morphology of the AISI 4340 steel thread roots

Fig.5 Surface roughness of the AISI 4340 steel thread roots

Fig.6 Surface hardness and residual stress of threaded roots

Fig.7 Microstructure of cross-section of AISI 4340 steel thread roots

Fig.8 Grain distribution in the cross-section of the thread root

Fig.9 Distribution of the local misorientation average angle of threaded root

Fig.10 Fracture morphology of AISI 4340 steel threads

Fig.11 Fatigue S-N curves of AISI 4340 steel bolts without rolling and with optimize rolling parameters

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