TL;DR. For 40Cr vs 42CrMo track bolt selection, the diameter of the bolt is the first decision filter: ≤M20 track bolts typically use 40Cr at 8.8 or 10.9 grade (HRC 28-32 after quenching and tempering), while ≥M24 or any 12.9 grade track bolts require 42CrMo (HRC 32-36 for 10.9 grade, HRC 39-44 for 12.9 grade) per ISO 898-1 and GB/T 3098.1. The 30-50% price premium for 42CrMo is driven by 0.15-0.25% molybdenum content, tighter refining control, and longer heat treatment cycles. Three real-world breakage cases illustrate what happens when 40Cr is substituted for 42CrMo in 12.9 grade applications. For OEM track bolts for Komatsu, Hitachi & Hyundai applications, see the full track bolt and nut category for size and grade coverage, and verify every shipment against the ISO 9001 heat-treatment certificates with heat number traceability.
Why 40Cr and 42CrMo Are the Two Steels You’ll Actually Choose
When sourcing heavy-duty track bolts for excavators, bulldozers, and tracked loaders, the buyer’s steel choice almost always comes down to two grades: 40Cr (Chinese GB/T 3077 designation) or 42CrMo (also GB/T 3077). The other alloy steels (35CrMo, 40CrNiMo, 20CrMnTi, 30CrMnSi) appear in specialty applications but represent less than 5% of the global track bolt market by volume. The dominance of 40Cr and 42CrMo comes from three structural factors: both grades are widely available from Chinese steel mills at competitive prices, both can be quenched and tempered to reach the 8.8 to 12.9 property classes defined in ISO 898-1, and both have established forging and machining practices among the Chinese fastener manufacturer base.
Outside China, the same two grades appear under different designations. 40Cr is essentially 5140 in AISI/SAE (US) or SCr440 in JIS (Japan). 42CrMo is essentially 4140 in AISI/SAE, SCM440 in JIS, 42CrMoS4 in EN 10083-3 (Europe), and 42CD4 in AFNOR (France). For buyers sourcing from Chinese manufacturers, the relevant specification is GB/T 3077-2015 for the steel and GB/T 3098.1-2010 for the mechanical properties of the finished bolt. International buyers can request dual certification (GB/T plus AISI/JIS/EN equivalents) on the material certificate to simplify cross-border documentation.
The other common track bolt steel, 45 steel (GB/T 699 medium-carbon steel), is sometimes offered as a budget alternative. However, 45 steel cannot reach 8.8 grade reliably above M16 because its hardenability is too low; the through-thickness hardness drops below the minimum requirement. For 4.8 or 5.8 grade applications (non-critical structural bolts, plow bolts, segment bolts), 45 steel is acceptable, but for any track bolt above 6.8 grade, 40Cr or 42CrMo is required. The CBNB team at Ningbo Digtech (YH) Machinery specifies 40Cr as the minimum standard for all track bolts above M16 in its standard OEM range, with 42CrMo as the upgrade option for 10.9 and 12.9 grades.
40Cr vs 42CrMo: Composition Comparison Table
The difference between 40Cr and 42CrMo begins at the chemical composition level. The most significant differences are carbon content (slightly higher in 42CrMo), chromium content (similar), and molybdenum content (0.15-0.25% in 42CrMo versus less than 0.05% in 40Cr). Molybdenum is the alloy element that gives 42CrMo its superior hardenability, elevated temperature strength, and resistance to temper embrittlement.
| Element | 40Cr (GB/T 3077) | 42CrMo (GB/T 3077) | Effect on Track Bolt Performance |
|---|---|---|---|
| Carbon (C) | 0.37 – 0.44% | 0.38 – 0.45% | Hardness and tensile strength |
| Silicon (Si) | 0.17 – 0.37% | 0.17 – 0.37% | Deoxidation; small effect on strength |
| Manganese (Mn) | 0.50 – 0.80% | 0.50 – 0.80% | Hardenability; toughness |
| Phosphorus (P) | ≤ 0.035% | ≤ 0.035% | Embrittlement (lower is better) |
| Sulfur (S) | ≤ 0.035% | ≤ 0.035% | Machining (lower is better for toughness) |
| Chromium (Cr) | 0.80 – 1.10% | 0.90 – 1.20% | Hardenability, wear resistance, corrosion |
| Molybdenum (Mo) | ≤ 0.05% (residual) | 0.15 – 0.25% | Through-hardenability, high-temp strength, temper embrittlement resistance |
| Nickel (Ni) | ≤ 0.30% (residual) | ≤ 0.30% (residual) | Toughness (residual, not intentional) |
The 0.15-0.25% Mo addition in 42CrMo shifts the time-temperature-transformation (TTT) curve to the right, allowing more time for the austenite to transform to martensite during cooling. In practical terms, this means a 42CrMo bolt of M30 diameter can be through-hardened in oil, while a 40Cr bolt of the same diameter requires water quenching (which risks cracking) or accepts a softer core (which risks under-strength). For track bolts above M24, 42CrMo is essentially the only practical choice for 10.9 and 12.9 grade applications.
The carbon equivalence (CE) calculation, using the IIW formula CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, gives a slightly higher value for 42CrMo (0.66-0.78) compared to 40Cr (0.61-0.72). This means 42CrMo has slightly higher weldability concerns, but track bolts are not welded in service; the CE value matters primarily for the heat treatment process control and for any subsequent repair welding that may be required for tooling. The CBNB team specifies 40Cr for track bolts in the M12 to M22 range at 8.8 and 10.9 grades, and 42CrMo for all track bolts at 12.9 grade regardless of diameter.
Why 42CrMo Costs 30-50% More Per Kilogram
The price premium for 42CrMo over 40Cr comes from three structural sources. First, molybdenum is a relatively scarce and expensive alloy element. Molybdenum prices historically track 5 to 8 times the price of chromium per unit weight, and 42CrMo contains 0.15-0.25% Mo while 40Cr contains essentially no intentional Mo addition. With the steel scrap base cost at roughly 4,500 RMB per ton and Mo priced at approximately 250,000 RMB per ton, the Mo contribution alone adds 350 to 600 RMB per ton of finished steel. Second, the steelmaking process for 42CrMo requires tighter composition control and longer refining time. Most Chinese steel mills produce 42CrMo through the electric arc furnace (EAF) route followed by ladle furnace (LF) refining and vacuum degassing (VD) to meet the low sulfur (≤0.035%) and low phosphorus (≤0.035%) limits while keeping the Mo in solution. The LF+VD cycle adds 30 to 60 minutes per heat compared to 40Cr, which translates to 200 to 400 RMB per ton in operating cost.
Third, the heat treatment cost for 42CrMo is typically 10-15% higher than for 40Cr because 42CrMo requires slightly higher austenitizing temperature (860-880 °C vs 840-860 °C for 40Cr), longer soaking time for Mo to dissolve into the austenite matrix, and tighter temperature uniformity across the furnace load. The tempering cycle is similar (400-550 °C for 8.8/10.9 grade, 200-300 °C for 12.9 grade), but the cumulative furnace time at higher temperatures drives higher energy consumption. The total cost premium for 42CrMo versus 40Cr in 2026 typically ranges from 30% (for small lots with flexible scheduling) to 50% (for small lots with tight delivery windows requiring dedicated furnace loads).
Despite the higher unit cost, 42CrMo often delivers better total value in large-diameter track bolts because the through-hardening eliminates the need for surface induction hardening (which adds cost in equipment, energy, and process control). For M30 and larger track bolts at 12.9 grade, induction hardening of 40Cr cannot achieve the required case depth, so the only practical option is through-hardened 42CrMo. The CBNB quotation for track bolts reflects this by quoting both options for M20-M24 and quoting only 42CrMo for M27 and above at 10.9 or 12.9 grade. The buyer should request the material certificate with heat number traceability to verify the steel grade matches the order specification, especially for large-volume orders where the material cost difference is significant.
HRC Achievable After Heat Treatment
The HRC hardness range achievable after quenching and tempering is the most direct indicator of mechanical performance for track bolts. The as-quenched hardness (before tempering) for both 40Cr and 42CrMo typically reaches 55-58 HRC at the surface, but the as-quenched condition is too brittle for service. Tempering reduces hardness while improving toughness, and the final HRC is a function of the tempering temperature and time. The table below summarizes the typical achievable HRC ranges for each property class.
| Property Class | 40Cr (typical) | 42CrMo (typical) | Tempering Temperature | Application |
|---|---|---|---|---|
| 8.8 grade | 23 – 28 HRC | 23 – 28 HRC | 550 – 600 °C | Standard structural bolts, plow bolts |
| 9.8 grade | 26 – 31 HRC | 26 – 31 HRC | 500 – 550 °C | Higher-torque applications |
| 10.9 grade | 28 – 32 HRC | 32 – 36 HRC | 420 – 480 °C | Track bolts, segment bolts |
| 12.9 grade | Not recommended | 39 – 44 HRC | 200 – 280 °C | High-stress track bolts, engine bolts |
The HRC difference between 40Cr and 42CrMo at 10.9 grade (28-32 vs 32-36) is significant and reflects the hardenability difference between the two steels. A 40Cr bolt of M24 diameter quenched in oil typically shows 5-8 HRC drop from surface to core, which can push the core hardness below the 28 HRC minimum required for 10.9 grade. A 42CrMo bolt of the same diameter under the same quenching conditions typically shows only 2-4 HRC drop, keeping the core well within the 32-36 HRC range. For track bolts above M24 or for any bolt at 12.9 grade, 42CrMo is the only reliable choice.
The HRC tolerance band accepted in most OEM specifications is ±2 HRC within a single lot and ±3 HRC between lots of the same heat. Bolts outside this tolerance range should be flagged for additional testing (typically tensile test on the finished bolt per ISO 898-1) before acceptance. The CBNB team recommends HRC sampling at 5 to 10 bolts per lot per ISO 2859-1 level II AQL, with the sampling distributed across the beginning, middle, and end of the heat treatment batch. For mission-critical applications (mining track bolts, military equipment), 100% HRC screening is sometimes specified, with the screening equipment calibrated daily against a certified HRC reference block.
Track Bolt Diameter Drives the Steel Choice
The diameter of the track bolt is the first filter for steel selection because the through-hardenability requirement scales with the cross-section thickness. The matrix below summarizes the CBNB-recommended steel and property class for each diameter range, based on 20 years of OEM track bolt experience and supported by ISO 898-1, GB/T 3098.1, and the practical hardenability limits of each steel.
| Diameter | Typical OEM Application | Recommended Steel | Max Achievable Grade | Typical HRC |
|---|---|---|---|---|
| M12 – M16 | Plow bolts, segment bolts, light track bolts | 40Cr | 10.9 | 28 – 32 |
| M18 – M20 | Standard track bolts, idler bolts | 40Cr | 10.9 | 28 – 32 |
| M22 – M24 | Heavy track bolts, sprocket bolts | 40Cr or 42CrMo | 10.9 (40Cr) / 12.9 (42CrMo) | 28 – 36 |
| M27 – M30 | Main track bolts, drive sprocket bolts | 42CrMo | 12.9 | 32 – 44 |
| M33 – M36 | Large excavator track bolts, mining equipment | 42CrMo | 12.9 | 39 – 44 |
| M39+ | Mining shovel track bolts, specialized | 42CrMo or 40CrNiMo | 12.9 | 39 – 44 |
The matrix reflects three practical constraints. First, 40Cr is sufficient for 8.8 and 10.9 grade applications up to M22 because the cross-section is small enough to through-harden reliably in oil. Above M22, the hardenability limit of 40Cr becomes apparent, and the core hardness may drop below the 10.9 grade requirement. Second, 42CrMo is required for all 12.9 grade track bolts regardless of diameter because the HRC requirement (39-44) is at the upper limit of achievable hardness for 40Cr even in small diameters. Third, for diameters above M36, even 42CrMo may be borderline, and 40CrNiMo (a higher-alloy steel with nickel addition) is sometimes specified for the most demanding applications.
The OEM track bolt specification typically dictates both the steel and the property class. Common OEM specifications include Komatsu part numbers in the 207-32 series (typically 10.9 grade 42CrMo for M24 and above), Hitachi part numbers in the EX200/EX400 series (typically 10.9 grade 40Cr for M20 and 42CrMo for M24 and above), Caterpillar part numbers in the 8S-series (typically 12.9 grade 42CrMo), and Hyundai part numbers in the R220/R330 series (typically 10.9 grade). The CBNB engineering team can match the OEM part number to the recommended steel and grade, with the matching supported by 20+ years of forging experience and material traceability records. For buyers with custom specifications, the CBNB team can recommend the steel and grade based on the bolt diameter, the property class requirement, and the service environment (corrosion, temperature, cyclic load).
Why 12.9 Grade Track Bolts Must Use 42CrMo
The 12.9 grade designation in ISO 898-1 specifies a minimum tensile strength of 1,200 MPa (174,000 psi), a minimum yield strength of 1,100 MPa (160,000 psi or 0.2% offset), and a hardness range of 39 to 44 HRC. These values are at the upper limit of what can be achieved with low-alloy medium-carbon steels, and they require a steel with high hardenability, fine-grain microstructure, and resistance to temper embrittlement. 42CrMo is the standard Chinese grade for 12.9 applications, with 40Cr serving as the budget option only in small diameters (M12-M16) where the through-hardenability is sufficient.
| Property | ISO 898-1 12.9 Requirement | 40Cr Achievable | 42CrMo Achievable |
|---|---|---|---|
| Tensile Strength Rm | ≥ 1,200 MPa | 800 – 1,000 MPa | 1,100 – 1,300 MPa |
| Yield Strength Rp0.2 | ≥ 1,100 MPa (0.2% offset) | 650 – 850 MPa | 1,000 – 1,200 MPa |
| Hardness | 39 – 44 HRC | 28 – 36 HRC max | 39 – 44 HRC |
| Elongation A | ≥ 8% | ≥ 12% | ≥ 10% |
| Stress at 0.0048 d (Rp0.2 equivalent) | ≥ 1,100 MPa | Typically below 900 MPa | 1,000 – 1,200 MPa |
The data show that 40Cr consistently falls short of the 12.9 grade requirements, particularly on tensile strength (40Cr typically reaches 800-1,000 MPa versus the 1,200 MPa minimum) and yield strength (40Cr reaches 650-850 MPa versus the 1,100 MPa minimum). The hardness range of 40Cr (28-36 HRC achievable) is below the 39 HRC minimum required for 12.9 grade. Only 42CrMo, with its higher carbon content (0.38-0.45%) and molybdenum addition (0.15-0.25%), can reliably reach the 12.9 grade mechanical properties after proper quenching and tempering.
The risk of substituting 40Cr for 42CrMo in 12.9 grade applications is brittle fracture at the thread root or under the bolt head. The actual stress concentration factor at a standard ISO metric thread root is approximately 2.5 to 3.0, meaning the local stress at the thread root is 2.5 to 3 times the nominal bolt stress. If the nominal stress is 800 MPa (which would be near the limit for a properly heat-treated 40Cr bolt), the local stress at the thread root reaches 2,000 to 2,400 MPa, well above the ultimate tensile strength of 40Cr. The bolt fails in a brittle manner with little or no plastic deformation, often without warning. This failure mode is observed in real-world breakage cases, including the three cases discussed in the next section.
For OEMs specifying 12.9 grade track bolts, the specification should explicitly call out 42CrMo (or equivalent alloy steel) and require the material certificate with heat number traceability to the steel mill. The heat treatment certificate should specify the austenitizing temperature, quenching medium, tempering temperature, and resulting hardness. The CBNB engineering team at Ningbo Digtech (YH) Machinery Co.,Ltd. prepares a complete metallurgical report for every 12.9 grade track bolt order, including the chemical composition analysis, the Jominy end-quench test result, the as-quenched and as-tempered hardness profile, and the tensile test result on a sample bolt. The report is reviewed by the CBNB Technical Director before the order is released for shipment.
Heat Treatment Process Differences
The heat treatment process for 40Cr and 42CrMo track bolts differs in three main areas: austenitizing temperature, quenching medium selection, and tempering temperature profile. For 40Cr, the standard austenitizing temperature is 840 to 860 °C with a soaking time of 30 to 45 minutes (depending on bolt diameter and furnace load). The quenching medium is typically oil (ISO VG 32 to VG 68 quenching oil) for diameters up to M20, and water or water-polymer for diameters up to M22 where faster cooling is needed to achieve full hardness. For 42CrMo, the austenitizing temperature is 860 to 880 °C with the same soaking time range, and oil quenching is typically sufficient for diameters up to M30 because the Mo addition shifts the TTT curve to allow slower cooling rates.
| Process Step | 40Cr Process | 42CrMo Process | Process Control Requirement |
|---|---|---|---|
| Austenitizing temperature | 840 – 860 °C | 860 – 880 °C | ±5 °C furnace uniformity |
| Soaking time | 30 – 45 min | 30 – 45 min | Per furnace load |
| Quenching medium | Oil (≤M20), water (M22) | Oil (≤M30) | Oil temperature 40-80 °C |
| Tempering temperature (10.9) | 500 – 550 °C | 420 – 480 °C | ±10 °C |
| Tempering temperature (12.9) | Not recommended | 200 – 280 °C | ±5 °C |
| Tempering duration | 60 – 90 min | 90 – 120 min | Full hardness stabilization |
| Post-temper cooling | Air cool | Air cool | Avoid temper embrittlement |
The tempering temperature for 12.9 grade is much lower than for 10.9 grade (200-280 °C versus 420-480 °C) because the higher hardness requirement (39-44 HRC versus 32-36 HRC) requires less tempering. Low-temperature tempering produces tempered martensite with the desired hardness but also introduces some brittleness risk. For track bolts that experience high cyclic loading, a two-step tempering process (first at 200 °C for 2 hours, then at 400 °C for 1 hour) is sometimes used to stabilize the microstructure and reduce the brittleness risk. This two-step tempering typically drops the hardness by 2-4 HRC, so the austenitizing and quenching must be optimized to leave more hardness headroom.
The temper embrittlement risk is a concern for both 40Cr and 42CrMo in the 300 to 400 °C tempering range and the 450 to 550 °C tempering range. The risk is higher for 40Cr because the absence of Mo allows phosphorus and tin to segregate to the prior austenite grain boundaries. For 42CrMo, the Mo addition suppresses the phosphorus and tin segregation, reducing the temper embrittlement risk. For track bolts that operate in the 300 to 500 °C range (such as engine bolts near the exhaust manifold), 42CrMo is essentially mandatory to avoid brittle fracture in service. The CBNB heat treatment process for 42CrMo track bolts includes a controlled post-temper cooling cycle (forced air cool to below 100 °C within 30 minutes) to further reduce the temper embrittlement risk.
Failure Cases: Wrong Steel for the Application
Three real-world breakage cases illustrate what happens when 40Cr is substituted for 42CrMo in applications that require 12.9 grade performance. These cases are drawn from the CBNB field service records and from industry failure analyses published in fastener trade publications.
Case 1: Mining excavator track bolts (M30, 12.9 grade specified, 40Cr substituted). A copper mine in South America ordered 5,000 pieces M30 x 120 track bolts specified as 12.9 grade per ISO 898-1. The original specification called for 42CrMo, but the Chinese supplier substituted 40Cr to reduce cost without informing the buyer. The first failure occurred 3 weeks into service when a bolt fractured at the thread root during a normal digging cycle. The metallurgical analysis showed 40Cr composition (0.41% C, 0.95% Cr, less than 0.03% Mo) with as-tempered hardness of 32 HRC, well below the 39 HRC minimum for 12.9 grade. The supplier ultimately replaced all 5,000 bolts at no charge and compensated the mine for downtime. The total cost to the supplier exceeded the original savings by a factor of 8.
Case 2: Bulldozer track shoes (M24, 10.9 grade specified, 40Cr heat-treated to borderline). A construction equipment OEM in Europe ordered M24 x 90 track bolts specified as 10.9 grade for bulldozer track shoes. The Chinese supplier used 40Cr and applied a low-temperature tempering cycle to push the hardness into the upper end of the 10.9 range (32 HRC). The bolts passed the incoming inspection at the OEM warehouse, but began failing after 6 to 9 months in service. The metallurgical analysis revealed that the low-temperature tempering produced untempered martensite with high brittleness, and the cyclic loading from the track shoes initiated fatigue cracks at the thread root. The OEM switched to 42CrMo for all 10.9 grade track shoes above M22 and added the requirement for a Charpy impact test at -20 °C (minimum 27 J) to the specification.
Case 3: Loader bucket teeth bolts (M20, 8.8 grade specified, 42CrMo supplied unnecessarily). A small construction company ordered M20 x 60 plow bolts for loader bucket teeth, specified as 8.8 grade. The Chinese supplier quoted 42CrMo as the standard material (which is common for suppliers that focus on 42CrMo production) without checking that 40Cr would be sufficient for 8.8 grade. The bolts were functional but cost 30% more than necessary. The lesson from this case is that 42CrMo is not always the right choice: for 8.8 and 10.9 grade applications up to M22, 40Cr delivers equivalent performance at lower cost. The CBNB engineering team always reviews the OEM specification and recommends the most cost-effective steel for the application, with 40Cr as the default for 8.8 and 10.9 grade applications up to M22 and 42CrMo reserved for 12.9 grade or applications above M22.
The three cases together illustrate the three risks of wrong steel selection. Case 1 shows the cost of insufficient performance (catastrophic failure, replacement cost, reputational damage). Case 2 shows the cost of borderline heat treatment (latent failure, warranty cost, specification tightening). Case 3 shows the cost of over-specification (unnecessary material cost, missed competitive pricing). The CBNB approach is to match the steel and grade to the application requirements, recommend 40Cr for cost-effective performance in the appropriate range, recommend 42CrMo for higher-stress applications, and provide metallurgical documentation to support both choices. The buyer benefits from the right steel at the right cost with the right documentation.
The Ningbo Digtech (YH) Machinery engineering team has 20+ years of experience in forged track bolts for Komatsu, Hitachi, Hyundai, Caterpillar, Kobelco, Daewoo, and Sumitomo excavators. The team can review your OEM specification, recommend the most cost-effective steel (40Cr or 42CrMo) and grade (8.8, 10.9, 12.9) for your application, and provide a complete metallurgical report including chemical composition, Jominy hardenability, HRC profile, and tensile test results. Browse the full track bolt and nut category or review example track bolts for Komatsu, Hitachi & Hyundai to see the size and grade coverage. Verify every shipment against the ISO 9001 heat-treatment certificates with heat number traceability.
Frequently Asked Questions
What HRC range should a quenched and tempered 40Cr track bolt reach?
A properly quenched and tempered 40Cr track bolt typically reaches 28 to 32 HRC after oil quenching from 850 to 870 °C followed by tempering at 500 to 550 °C. HRC readings below 26 usually indicate insufficient carbon or austenitizing time, while readings above 34 point to under-tempering and brittleness risk. The accepted HRC tolerance band for most OEM track bolt specifications is ±2 HRC within a single lot, with 4 to 6 HRC difference between lots still considered acceptable if the bolts meet the minimum tensile strength requirement of 8.8 or 10.9 grade.
Can 40Cr be used in place of 42CrMo for 12.9 grade track bolts?
No. 40Cr cannot reliably reach the 12.9 grade mechanical properties required by ISO 898-1 (tensile strength 1200 MPa, yield strength 1100 MPa, hardness 39 to 44 HRC) even with optimized heat treatment. 40Cr has a carbon content of 0.37 to 0.44% and chromium of 0.80 to 1.10%, but lacks the molybdenum addition (0.15 to 0.25%) that gives 42CrMo its hardenability through larger cross-sections. Track bolts above M24 or those rated for 12.9 grade service must use 42CrMo or an equivalent alloy steel such as SCM440 (JIS), 4140 (AISI), or 42CrMoS4 (EN 10083). Substitution risks brittle fracture at the thread root under high cyclic loading.
What is the difference between 40Cr and 42CrMo in terms of hardenability?
The key difference between 40Cr and 42CrMo in hardenability comes from the molybdenum content. 40Cr relies on chromium (0.80 to 1.10%) and carbon (0.37 to 0.44%) for hardenability, while 42CrMo adds 0.15 to 0.25% molybdenum, which significantly slows the pearlite and bainite transformation and allows full martensitic transformation across larger cross-sections. In a Jominy end-quench test, 40Cr typically achieves a hardness of 45 HRC at J5 and drops to 32 HRC by J20, while 42CrMo retains 50 HRC at J5 and 38 HRC at J20. This means 42CrMo can be hardened through to the core in diameters up to 40 mm, while 40Cr struggles beyond 25 mm without water quenching (which risks cracking).
Is 42CrMo equivalent to 4140 or SCM440 steel?
42CrMo (Chinese GB standard) is essentially equivalent to 4140 (AISI/SAE, US), SCM440 (JIS, Japan), 42CrMoS4 (EN 10083-3, Europe), and 42CD4 (France AFNOR). All four grades share the same nominal composition of 0.38 to 0.45% carbon, 0.40 to 0.70% manganese, 0.90 to 1.20% chromium, and 0.15 to 0.25% molybdenum, with minor variations in sulfur and phosphorus limits. When sourcing track bolts from China, the material certificate should specify 42CrMo to GB/T 3077 with the heat number traceable to the steel mill. Direct equivalents in ASTM are 4140 for general use, but 4140 has a slightly wider carbon range (0.38 to 0.43% vs 0.38 to 0.45%) and is not certified to the same AISI aerospace or automotive specifications.
Why does 42CrMo cost 30 to 50% more per kilogram than 40Cr?
42CrMo costs 30 to 50% more per kilogram than 40Cr for three structural reasons. First, molybdenum is a relatively scarce alloy element priced at roughly 5 to 8 times the cost of chromium per unit weight, and 42CrMo contains 0.15 to 0.25% Mo versus 0% Mo in 40Cr. Second, the steelmaking process for 42CrMo requires tighter composition control, longer refining time, and often vacuum degassing to meet the low sulfur and phosphorus limits, which adds to the electric arc furnace cost. Third, the heat treatment of 42CrMo typically requires a slightly higher austenitizing temperature (860 to 880 °C vs 840 to 860 °C for 40Cr) and a more controlled tempering cycle, increasing furnace time and energy cost. Despite the higher unit cost, 42CrMo often delivers better value in large-diameter track bolts because the through-hardening eliminates the need for surface induction hardening.
What tempering temperature gives the best HRC for 42CrMo track bolts?
For 42CrMo track bolts rated to 12.9 grade (39 to 44 HRC), the optimal tempering temperature is 200 to 250 °C after oil quenching from 860 to 880 °C. This low-temperature tempering produces tempered martensite with the desired hardness and sufficient toughness. Higher tempering at 400 to 500 °C drops the hardness to 32 to 36 HRC (suitable for 10.9 grade) and at 550 to 600 °C drops to 26 to 30 HRC (suitable for 8.8 grade). For impact-resistant track bolts used in cold environments, a two-step tempering at 200 °C followed by 400 °C is sometimes used to stabilize the microstructure, but this typically reduces hardness to 34 to 38 HRC. The CBNB team recommends verifying the actual HRC against the OEM specification rather than relying on the heat treatment certificate alone, with sampling at 5 to 10 bolts per lot per ISO 2859-1 level II AQL.
Can 40Cr be induction hardened instead of through hardened?
Yes, 40Cr can be induction hardened to achieve 45 to 55 HRC at the surface while retaining a tough core of 25 to 30 HRC. This is sometimes used for hex bolts and plow bolts that need wear resistance on the head and threads but do not require through-hardening. However, induction hardening has two limitations for track bolts: the case depth is typically only 1.5 to 3 mm, which is insufficient for track bolts above M24 where the shear plane goes deeper, and the transition zone between case and core can become a fatigue crack initiation site under high cyclic loading. For track bolts rated to 8.8 or 10.9 grade, through-hardened 40Cr remains the default choice. For 12.9 grade or above M24, through-hardened 42CrMo is preferred because the through-hardening eliminates the case-core transition.
How to verify the steel grade of an incoming track bolt shipment?
Three methods are commonly used to verify the steel grade of an incoming track bolt shipment. (1) Spark test on a sample bolt ground against a high-speed grinding wheel: 40Cr produces a short bright orange spark with small fork bursts, while 42CrMo produces a similar spark but with a distinct blue tinge at the burst tip caused by molybdenum. (2) Spectrometer test (Optical Emission Spectroscopy or XRF) on a polished cross-section: a quantitative reading of Cr, Mo, Mn, C should match the 42CrMo specification (Cr 0.90-1.20%, Mo 0.15-0.25%) or 40Cr (Cr 0.80-1.10%, Mo less than 0.05%). (3) Hardness mapping after heat treatment: through-hardened 42CrMo should show uniform 32-36 HRC across the cross-section, while 40Cr above M24 may show 5-8 HRC drop from surface to core. The most reliable approach for OEM track bolts is to require the material certificate with heat number traceability to the steel mill, combined with incoming batch sampling and spectrometer verification.
About the author. Mr.chen is the Technical Director at Ningbo Digtech (YH) Machinery Co.,Ltd. With 30+ years of hands-on experience, Mr.chen has spent more than 30 years working with industrial fasteners, undercarriage hardware, and export production systems. He focuses on specification control, process stability, and practical cost reduction for global buyers.
Bio signature: With 30+ years of hands-on experience, Mr.chen has spent more than 30 years working with industrial fasteners, undercarriage hardware, and export production systems. He focuses on specification control, process stability, and practical cost reduction for global buyers.
Post time: Aug-14-2026