Why Heat Treatment Decides SDS Body Performance
An SDS drill body has to transmit impact, carry torque, guide debris and survive repeated chuck loading. Machining defines the geometry; heat treatment determines whether that geometry has the hardness and toughness to keep working under impact.
The objective is not maximum hardness everywhere. A uniformly over-hard body becomes fracture-prone; a body that is too soft deforms, wears at the shank and loses flute or gauge stability. The approved heat-treatment route must match the steel grade, diameter, section changes and impact duty.
The Zhonghuan Isothermal Salt-Bath Route
One confirmed Zhonghuan route for SDS steel bodies is isothermal salt-bath heat treatment. The workpiece is heated and transferred through controlled molten-salt stages so the full surface receives rapid, uniform heat transfer. This is materially different from heating in open air and judging the part by colour.
Salt contact around flutes, lands and shank features helps reduce local hot and cold zones. The isothermal stage gives the cross-section time to equalize and transform within the approved window before the next cooling or tempering step. Exact bath chemistry, temperatures and hold times are process parameters tied to the approved alloy and geometry; they are not one universal public recipe.
The Zhonghuan Controlled-Atmosphere Route
Zhonghuan also supports controlled-atmosphere heat treatment for programs assigned to an enclosed furnace route. Here the atmosphere, oxygen exposure, furnace-zone curve, residence time, load density and cooling sequence are managed together to limit oxidation and decarburization while holding the approved thermal history.
Controlled atmosphere and isothermal salt bath are not competing marketing tiers. They are different process routes selected by steel, section, geometry, surface requirement and volume program. The purchase specification should name the required result and evidence; Zhonghuan assigns the approved route.
The Process Window That Must Be Locked
| Control | Why it matters |
|---|---|
| Steel grade and incoming condition | Sets transformation response and the available hardness-toughness window. |
| Diameter and section change | Controls how quickly the core follows the surface and where distortion can concentrate. |
| Transfer time | Limits uncontrolled cooling between stages. |
| Bath temperature and hold | Defines the isothermal transformation window and cross-section uniformity. |
| Load density and circulation | Prevent one packed area of the batch from seeing a different thermal history. |
| Tempering and final cooling | Relieve stress and tune the final balance for the product duty. |
Changing steel, diameter, flute mass or shank section can require a revised cycle. Copying one timer setting across the full SDS-Plus, SDS-Max and chisel range is not process control.
Hardness and Toughness by Function
The flute body, transition and SDS connection do different jobs. The body carries torque and impact; the shank repeatedly contacts chuck components; section transitions must avoid becoming fatigue notches. Zhonghuan therefore releases hardness against defined inspection zones and the approved product specification, not one headline HRC value for the entire tool.
Carbide grade and carbide-to-steel joining are separate from steel-body heat treatment. The salt-bath route establishes the steel condition first; induction brazing, controlled-atmosphere brazing or resistance friction welding must then preserve the approved body performance around the connection zone.
Distortion, Straightness and Runout
Flutes, long working lengths and sharp section changes make SDS bodies sensitive to thermal distortion. Uniform bath heat transfer and an isothermal hold reduce gradients, but heat treatment does not eliminate the need for straightening and measurement.
Zhonghuan places straightness correction and runout inspection after thermal processing in the manufacturing flow. Acceptance is tied to bit length, diameter, interface and the approved drawing; a long SDS-Max mining bit and a short SDS-Plus bit do not share one practical straightness plan.
Quality Evidence and Batch Release
- Cycle record: approved recipe, bath identification, time record and batch traceability.
- Hardness by zone: measurement locations and acceptance range defined by the specification.
- Straightness and runout: checked after cooling and correction.
- Representative section or microstructure: added when first-article or project risk requires it.
- Impact and drilling validation: representative samples tested with the target hammer class and substrate.
Not every shipment needs every destructive test. The first-article plan establishes the evidence set, and routine batch release retains the controls needed to hold that approved condition.
What to Put in an OEM Specification
Send the SDS interface, body-steel requirement, diameter and working length range, head construction, hammer class, target substrate, critical hardness zones, straightness/runout requirement and sample-approval plan. Zhonghuan maps those buyer inputs to the internal isothermal salt-bath cycle and inspection route.
Continue with the manufacturing archive and the SDS joining-method guide to see how heat treatment and carbide joining remain separate but coordinated controls.
