The Influence of Microcracks Generated During Forging on Crack Propagation in Steel Forgings

Abstract

This article investigates the formation of solidification cracks in steel forgings used for bearing rings in gear reducers of robotic arms. The forging and heat treatment processes, conducted under consistent technological conditions, revealed the occurrence of high-temperature annealing cracks caused by plasticity depletion during stress relaxation. Additionally, solidification cracks were analyzed, with chemical compositions and hardness measurements indicating susceptibility due to elevated carbon and chromium content, as well as a high cracking parameter. Die tool wear and damage during forging were identified as key contributors to crack formation, transferring surface defects, inclusions, and creating cracks that propagate during subsequent processing. The findings underscore the influence of the tooling conditions, material properties, and process parameters on the quality and reliability of steel forgings.

1. Introduction

The manufacturing of steel bearing rings for gearbox components is highly demanding in terms of adhering to technological processes to achieve the desired product quality [1]. The final product is influenced by specific technological procedures and the composition of the input material [2]. In standard production, it is challenging to detect latent defects in products during processing. In most cases, defects are identified only during final quality control, requiring subsequent analysis to determine their root causes.

Defect identification in forgings is feasible only through categorization of the defects. Crack-type defects are particularly challenging to identify due to the wide range of factors contributing to their formation. George E. Dieter, in his works published in the ASM Metals Handbook [3,4], categorized cracks based on their temperature of origin during forging and their type—taking into account orientation relative to the forging axis, location on the forged part, and the forging method used.

A fundamental classification of surface crack types is described in the EN 1011-2 standard [5]. While this standard primarily addresses cracks arising during welding, it is not limited to those occurring in welds but also includes cracks in the heat-affected zones of the parent metal. According to this standard, base materials include not only sheets but also pipes and forgings.

Researchers Viňáš, Brezinová, Maňková, and Brezina [6] adopted the basic classification of cracks from the EN 1011-2 standard and further refined it into four primary groups, each with subcategories. These groups include the following: Hot cracks, such as solidification, liquation, and polygonization cracks. Cold cracks, a broad category without subdivisions. Lamellar cracks, divided into exogenous and endogenous types, and annealing cracks.

Solidification cracking has been a subject of study for many researchers [7,8]. Ito and Bessyo proposed a formula and established limits for structural low-alloy steels with higher manganese content (0.8–2.5%) [9]. They introduced an equation for calculating the cracking parameter of the parent metal, denoted as P_C, and also identified a critical threshold of P_C > 0.30% for material thicknesses of 25 and 30 mm, indicating a high probability of cracking. Furthermore, they defined permissible content levels for individual alloying elements to minimize crack formation. Their studies showed that cracks were absent in samples with a maximum hardness up to 271 HV = 264 HB.

The focus of this paper will be on solidification cracks on forging. Since the steel 42CrMo4 heating temperature does not exceed 1220 °C in the forging process, the liquid phase is not reached [10]. Hot cracking and solidification cracking are often used interchangeably, but hot cracking can also include liquation cracks.

Solidification cracking during forging presents a significant challenge in the production of steel forgings, as surface defects can drastically affect the quality and mechanical properties of the forged components [11,12]. Such cracks may lead to product failure in applications subjected to high mechanical or thermal loads [13,14].

The aim of this study is to localize cracks occurring in the forged bearing rings and determine the underlying causes of their formation.

2. Materials and Methods

Empirical observations of defect occurrences were conducted on the product “Bearing ring” (Figure 1), designed for gearbox applications as a combined component of a bearing housing and a force transmission converter. The product features a simple geometric shape—a rotational cylinder with a central hole and a non-uniform cross-section.

The product dimensions are as follows: outer diameter—Ø 155 mm, inner diameter of the central hole—Ø 98 mm, height—63 mm, and the weight of the forging is approximately 4 kg.

The forging is manufactured from 42CrMo4 steel, classified according to ISO 683-2:2016 [15]. The material is normalized, annealed, and hardened to achieve the prescribed mechanical properties.

After machining operations, specific areas of the product undergo surface hardening to increase hardness, followed by grinding to reduce surface roughness.

The technological and production operations for the forging of the bearing ring are summarized in

Table 1. Technology operations overview (explanations: N—normalizing annealing, Q—quenching, QT—quenching and tempering).

Tech. Operation	Heating	Forging			Cooling	Heat Treatment
		Open-Die	Closed-Die	Shearing and Punching		Normalizing Annealing	Quenching	Tempering
Machine	Induction furnace	Mechanical press 25,000 kN		Mechanical press 5000 kN	Belt conveyor	Continuous gas furnace	Continuous gas furnace	Continuous electric furnace
Material	Ø = 90 mm, h = 132 mm	Ø = 102 mm, h = 80 mm	Forging with flash and slug	Final forging; 42CrMo4		42CrMo4 + N	42CrMo4 + N + Q	42CrMo4 + N + QT
Working temperature	1160–1220 °C	1100–1160 °C	1100–1160 °C	950–1160 °C	20–25 °C	870 ± 10 °C; air cooling	870 ± 10 °C; polymer cooling on 25 °C	660 ± 10 °C

. The heating of the billet is performed in a continuous induction furnace at a temperature range of 1160 to 1220 °C.

The forging of the “bearing ring” is carried out on a mechanical forging press with a maximum working force of 25,000 kN. Forging tools for this operation are designed to combine two stages of forging (Figure 2a):

• Pre-forging (open-die): This phase involves reducing the billet to the desired height of the preform and shaping it to prepare for the next phase.
• Final forging (closed-die): This phase involves filling the forging cavity to achieve the desired shape of the forging. The result includes a slug for the central hole and flash along the edges of the forging.

Subsequently, on a separate machine—a mechanical press with a maximum working force of 5000 kN—the forging (1) undergoes slug (2) and flash (3) removal. In the first phase, a punching pin removes the slug-membrane from the inner hole of the forging. In the second phase, a cutting plate is used to trim the flash, achieving the final shape and dimensions of the forging (Figure 2b).

After forging, the product undergoes a heat treatment process consisting of normalization annealing, quenching, and tempering. Normalization annealing is carried out in continuous gas furnaces with electronically controlled temperature systems that comply with the DIN 17052-1 [16] standard (requirements for temperature uniformity). The process is conducted at a temperature of 870 °C with a holding time of 30 min.

Quenching is performed in a continuous gas furnace, also at a temperature of 870 °C, followed by immersion into a synthetic polymer quenching medium at 25 °C for a soaking time of 30 s. Subsequently, the products pass through a continuous electric tempering furnace at a temperature of 660 °C with a holding time of 2.5 h.

Once cooled, the forgings are shot-blasted in a shot blasting machine to remove surface impurities and prepare them for final quality control.

The forgings were subjected to non-destructive testing. The first phase involved visual inspection according to EN 13018:2016 [17], followed by magnetic particle inspection in the second phase, performed in accordance with EN 10228-1:2016 [18].

To assess the mechanical properties, conduct chemical analyses, and perform metallographic observations, a cross-section of the test sample was prepared.

For chemical composition analysis, the method of optical emission spectroscopy was employed (SPECTRO Analytical Instruments GmbH, Kleve, GERMANY). The chemical composition of individual batches is presented in

Table 2. Chemical analysis of individual batches (wt%, Fe balance).

Chemical Element	Batch: 63813 (wt%)	Batch: T21297 (wt%)
C	0.38	0.43
Si	0.22	0.29
Mn	0.80	0.75
P	0.013	0.011
S	0.010	0.003
Cr	1.10	1.14
Mo	0.214	0.203
Al	0.023	0.022
Cu	0.17	0.02
V	0.0044	0.004
Ni	0.17	0.03

. The results of the elemental analyses fall within the specified limits for 42CrMo4 steel.

To determine the P_C cracking parameter, the method according to Ita and Bessy was used, which combines the amount of individual chemical elements in the steel composition, the thickness of the steel product (t), and the amount of diffusible hydrogen (H):

$$P_C = C+{\displaystyle \frac{Si}{30}}+{\displaystyle \frac{Mn}{20}}+{\displaystyle \frac{Cu}{20}}+{\displaystyle \frac{Ni}{60}}+{\displaystyle \frac{Cr}{20}}+{\displaystyle \frac{Mo}{15}}+{\displaystyle \frac{V}{10}}+5B+{\displaystyle \frac{t}{600}}+H/600$$

(1)

The Charpy impact test was used to evaluate the impact toughness, in accordance with EN ISO 148-1:2016 [19]. The results of the impact toughness tests are presented in

Table 3. Impact test results.

Batch	KV + 20 °C (J)
	Probe 1	Probe 2	Probe 3
63813	118.0	108.9	109.5
T21297	111.2	110.7	105.0

, meeting the required limit (KV + 20 °C, minimum 35 J) for 42CrMo4 steel in the N + QT condition.

The Brinell hardness test was conducted in accordance with EN ISO 6506-1:2014 [20]. The measured surface hardness for batch 63813 is 278 HB, and for batch T21297, it is 272 HB. The specified hardness for the tested forging (bearing ring) is in the range of 240–270 HB.

3. Results

Two types of non-destructive testing (NDT) were performed during the final quality control: Visual Testing (VT) and Magnetic Particle Testing (MT). An analysis was conducted on batch number 63813, consisting of 534 pieces, and batch number T21297, consisting of 625 pieces. The number of defective forgings after each inspection phase is presented in

Table 4. Number of forgings from individual batches by type of non-destructive testing.

Batch	Number of Forgings (pc)	Pieces with a Crack (pc)		Pieces with a Crack from Good Pieces (%)
		VT Method	MT Method
63813	534	212	381	71.34
T21297	625	23	164	26.24

. Cracks were observed on both the outer and inner sides of the forgings.

The occurrence of cracks in forgings in individual batches is in different volumes: 71.34% in batch 63813 and 26.24% in batch T21297. Only forgings after MT are included in the error rate calculation.

Cracks were localized on both the outer side of the forging (Figure 3, position 4) and the inner surface of the hole (Figure 3, position 5). The length of the cracks ranged from 60 to 150 mm. These cracks were observed along the entire profile, most commonly starting and ending at the area where the flash is located. Their progression extended toward the radii at the outer side of the forging, where the edge of the forging thins.

Two zones of crack occurrence were identified, as described in

Table 5. Crack parameters and direction in forging zones.

Cracks Zones	Crack Parameter (mm)		Direction of Cracks
	Depth	Length
I Flash area	0–33	15–320	To the center
II Slug area	0–22	15–145	To the center

and marked in Figure 4. The orange marks indicate the cracks are in I. the outer flash area, while the blue marks indicate cracks are in II. the inner slug area.

The direction of the cracks is centric, from the edge, and directed to the center of the forging cross-section. On macroscopic observation of the crack—Figure 5 and Figure 6—an oxide layer is clearly visible along its entire length. The crack starts and stops at the point of denser fiber concentration on the surface of the forgings. The running of cracks on the surface is along the densest fiber concentration, or to the forging radius location and back to the densest fiber concentration.

Table 6. Calculating the cracking parameter P_C.

Chemical Composition	Batch 63813	Batch T21297
t	30 mm	30 mm
H	1 ppm	1 ppm
PC	0.57%	0.61%

shows the calculated cracking parameter P_C for individual batches based on the chemical composition and thickness of the parent metal, according to the method (1) of Ito and Bessyo. Cracks do not occur at values of the cracking parameter P_C below 0.30% for material thicknesses of 25 and 30 mm.

To evaluate the heat treatment process, a hardness test was performed on a cross-section of the forging. The individual measurement values are presented in

Table 7. Measured hardness (HB) values in the cross-section of the forging.

Line	1	2	3	4	5	6	7	8	9	10
A	272	272	266	272	266	266	266	272	272	278
B	272	272	266	266	255	266	266	272	272	278
C	-	-	-	272	272	272	-	-	-	-

. A network of measurement points was established, as shown in Figure 7. The surface hardness of the forging was similar for both batches, exceeding 270 HB. The lowest hardness was observed at the center of the forging cross-section. The difference between the surface hardness and the hardness at the center of the cross-section was 22 HB.

Tool wear was not monitored during forging. After forging the entire batches, the forging tools—specifically the die and trimming tools—were subjected to visual inspection and dies were repaired by milling. Both types of tools exhibited clear signs of erosive wear. A significant crack was observed on the upper die, spanning approximately three-quarters of the die’s circumference, as illustrated in Figure 8b (marked in the red zone). Dimensionally, the tools remained within the tolerance limits. On the punch, piercing tool, and trimming plate, substantial wear on the cutting edges and radial cracks in hard metal welds were identified, marked in orange in Figure 8a.

4. Discussion

Based on the evaluation of the test results and laboratory analyses, it can be concluded that the localization, direction, and size of the cracks were identical in both batches. These are high-temperature annealing cracks with brittle fractures that formed during the quenching process of the forgings. The root cause of these cracks is the depletion of plasticity in the critical zone of the heat-affected area during heat treatment, specifically during the relaxation of residual stresses [6].

It is important to note that these high-temperature annealing cracks are merely a consequence of intergranular discontinuities transferred from the forging process in the austenitic phase, as illustrated in Figure 5. The hot forging die failure is complicated due to various kinds of influencing variables such as the die material, die design, die manufacturing, and forging operations. It is also necessary to note that defects in the forging tools—such as cracks and surface irregularities—can be transferred to the forging itself (indicated by the orange zones), leading to the formation of hot microcracks. The solidification phases for research material are typically austenite and austenite + perlite and the ending phase is sorbite [21,22,23].

During the trimming and piercing operations, mechanical removal of a portion of the metal from the wall surface occurs via shearing [24]. This process subtly disrupts the structure of surface bonds, resulting in fractures during the austenite formation phase [25]. This can cause structural disharmony, leading to the development of microcracks, which act as precursors to solidification crack formation (indicated by the blue zones in Figure 9) during subsequent heat treatment processes, such as quenching.

Both batches were forged using the same technological procedure, on the same dies, and underwent identical heat treatment processes. The chemical composition of the melts is comparable, except for the copper (Cu) content, which is 0.17% in batch 63813 and 0.02% in batch T21297. The carbon (C) content exceeds 0.25%, and the chromium (Cr) content in both batches is above 0.9%, exceeding the recommended limit for the occurrence of solidification cracks [14].

The cracking parameter (P_C) was 0.57% and 0.61% for the two batches, both significantly above the threshold of 0.30%. The surface hardness exceeded 271 HB, and the hardness difference across the sample cross-section was greater than 22 HB, which are critical parameters that indicate a high likelihood of solidification crack formation.

5. Conclusions

The reasons for the incidence of hot cracks are many and complex, but in general terms, they occur when localized ductility is insufficient to support imposed strains. The lack of ductility can depend upon micro structural features and orientation (relative to the strains) and in some cases upon the presence of brittle impurities and low-melting-point (or liquated) films. In this respect some alloy systems are highly sensitive to the presence of impurity elements such as sulfur, phosphorus, lead, etc. Impurity levels influence the incidence of cracking in such structures [26]. Norm EN ISO 17641-1:2004 [27] describes that, “Precise mechanisms for the occurrence of hot cracking have not yet been fully established.”

Deformations in the shape of forging tools, such as wear or damage, have a significant impact on the formation of hot solidification cracks in forgings. During the forging process, tools experience continuous wear and shape changes, which can influence how forces are applied to the material and its deformation. If the tooling is worn or damaged, it may lead to localized inclusion clusters within the forgings or the transfer of surface defects, resulting in the development of cracks on the forging surface.

This issue can subsequently cause crack propagation or enlargement during subsequent technological processes or during product use. Such defects reduce the quality of the forgings and compromise their reliability in final applications.