Why Diameter Changes the Result of Quenching
Quenching works by cooling austenite fast enough to form martensite, and the cooling rate inside a bar depends on how far the heat must travel to reach the surface. A small diameter bar cools quickly through its whole cross-section, while a large diameter bar cools slowly in the centre. Because the core of a large bar cannot reach the critical cooling rate, it transforms to softer bainite or ferrite-pearlite instead of martensite, and the final strength after tempering is lower in the core than at the surface.
Typical Property Levels at Different Diameters
GB/T 3077 lists mechanical property requirements for 42CrMo tested on specimens taken from bars, and the achievable properties fall with diameter:
Small bars up to about 40 mm diameter can be hardened through and typically reach a tensile strength of 1080 MPa or more with yield above 930 MPa after oil quenching and tempering.
Medium bars of 40 to 100 mm show a measurable drop in core properties, with tensile strength typically settling in the 900 to 1080 MPa range depending on the tempering temperature.
Large bars over 100 mm develop a substantial soft core, and the specified minimums must be reduced or the steel must be quenched in water or a more severe quenchant with careful control.
These are general patterns; the exact values depend on the heat treatment, the test location and the acceptance criteria agreed between buyer and supplier.
The Hardenability Concept Behind the Effect
Hardenability is the ability of a steel to form martensite at depth, and it is measured by the Jominy end-quench test described in standards such as GB/T 225 or ASTM A255. The Jominy curve shows hardness as a function of distance from the quenched end, and it lets the heat treater predict what hardness a given diameter will reach with a chosen quenchant. For 42CrMo, chromium and molybdenum push the Jominy curve high and flat, which is exactly why the grade is chosen for medium-size shafts and gears where deep hardening is required.
Practical Heat Treatment Strategy
For a given diameter, the heat treater has three levers: the quench severity, the tempering temperature and the test location. Oil quenching is standard for 42CrMo because it balances hardening with distortion control, while polymer or water quenching increases depth of hardening for larger bars at the cost of higher distortion risk. Tempering temperature is then adjusted to hit the required strength and toughness: lower tempering gives higher strength with lower toughness, and higher tempering trades strength for ductility and impact resistance.
How to Specify 42CrMo Round Bars Correctly
When ordering 42CrMo round steel, state the diameter, the grade and the required condition such as quenched and tempered, and define the acceptance values and test location. For critical parts, require a Jominy hardenability band from the mill, specify the core hardness minimum, and agree on the sampling position, usually at a defined depth below the surface or at the centre for small bars. The Mill Test Report should document the heat number, chemistry, heat treatment and the mechanical test results for the actual bar size supplied.
Frequently Asked Questions
Q: Why does a large 42CrMo bar have lower strength than a small one?
The core of a large bar cools too slowly during quenching to form full martensite, so it transforms to softer microstructures and the strength after tempering is lower.
Q: Up to what diameter can 42CrMo be fully hardened?
With oil quenching, full hardening is practical up to roughly 40 mm diameter; larger sizes need water or polymer quenching and careful verification of core properties.
Q: What is a Jominy test?
The Jominy end-quench test measures hardenability by quenching one end of a standardized bar and reading hardness along its length, producing a curve that predicts depth of hardening for real sections.
Q: Does tempering change the size effect?
Tempering lowers the hardness of martensite but cannot fix a core that never formed martensite, so the diameter effect on core strength remains after tempering.
Q: How should acceptance testing be agreed for large bars?
The buyer should specify the test location, the minimum core hardness or strength, and the reference to GB/T 3077 or the equivalent specification so the supplier can test and report accordingly.