Why Heat Treatment Matters for Buyers
When you order precision steel strip, you are not just buying a dimension and a grade. You are buying a metallurgical condition. The same SK5 strip at 0.5 mm thickness can arrive soft enough to deep-draw or hard enough to spring back — the difference is heat treatment.
Heat treatment determines formability, hardness, fatigue life, and surface condition. If you specify the wrong delivery condition, your stamping dies wear faster, your springs fail early, or your blades won't hold an edge. Understanding what you're ordering saves tooling costs and avoids reject batches.
Delivery Conditions Explained
Annealed / Spheroidized
Annealed strip has been heated above its recrystallization temperature and slow-cooled to relieve internal stress. The result is a soft, ductile material that forms easily. Spheroidized annealing goes further: it converts the lamellar (plate-like) pearlite carbides into round, spheroidal particles dispersed in a ferrite matrix.
Why does this matter? Spheroidal carbides reduce resistance to plastic deformation. The strip bends without cracking, stamps without tearing, and feeds through progressive dies without galling. For high-carbon grades like SK5, SK7, and 65Mn, spheroidized annealing is essential before any cold forming operation.
Hardened & Tempered (H&T)
Hardened-and-tempered strip has already undergone quenching and tempering, but it is not automatically ready for every finished part. Buyers must confirm that the delivered properties, flatness, surface and permissible forming match the downstream process and component validation plan.
Annealed vs. Hardened & Tempered: At a Glance
| Property | Annealed / Spheroidized | Hardened & Tempered |
|---|---|---|
| Hardness (HV) | 150–220 | 380–620 |
| Formability | Excellent — deep draw, stamp, bend | Limited — slight bends only |
| Typical Use | Stamping blanks, saw blade bodies, forming stock | Springs, blades, scrapers, clips |
| Further Processing | Often hardened after forming | May reduce later heat treatment; finished-part validation still required |
| Surface Options | Bright, matte | Blue, gold, polished, matte |
| Common Grades | SK5, SK7, 65Mn, C75S, SKS51 | SK5, SK7, 65Mn, C75S, SKS51 |
What is Spheroidization?
Spheroidization is an annealing route intended to produce a more spheroidal carbide morphology. The applicable cycle depends on grade, prior microstructure, section size and equipment; a generic temperature range should not be used as a production instruction.
Where spheroidization matters, the order should define the metallographic method, rating or percentage criterion, sampling location and frequency. A grade name or the word “annealed” does not establish a batch-specific result.
Higher spheroidization generally improves formability by reducing continuous carbide networks, but the finished result also depends on grade, dimensions, prior processing and the customer's forming and heat-treatment conditions. Buyers should request metallographic evidence against an agreed acceptance criterion.
The Hardening & Tempering Process
Hardening generally involves austenitizing and quenching, followed by tempering. Exact temperatures, soak time, atmosphere and quench medium are grade-, dimension- and equipment-specific and should come from a validated process specification.
Tempering adjusts the balance of hardness and toughness. The examples below are conceptual ranges only, not instructions or guaranteed outcomes:
- 200–250°C: Maximum hardness retained (HV 580–620). Used for cutting tools, doctor blades, scraper blades.
- 280–320°C: Balanced hardness and elasticity (HV 480–540). Used for springs, bandsaw backing, textile reeds.
- 350–400°C: Higher toughness, moderate hardness (HV 380–450). Used for flexible springs, clip components, automotive parts.
Continuous and batch heat-treatment routes have different control and distortion considerations. Neither route should be judged from equipment type alone; buyers should define the required properties, test method, sampling plan and dimensional acceptance criteria.
Surface Finishes After Heat Treatment
Heat treatment changes the strip surface. The finish you receive depends on the atmosphere control during tempering and any post-treatment polishing:
- Blue (tempered blue): A thin oxide layer formed during tempering at ~300°C in controlled atmosphere. Provides mild corrosion resistance and a visual indicator of temper condition. Standard for bandsaw strip stock and general springs.
- Gold (straw/gold): Lower-temperature temper oxide (~220–250°C). Indicates a harder temper. Common on blade stock and high-hardness spring strip.
- Polished (bright): Ground or polished after heat treatment to remove all oxide. Required for applications where surface smoothness matters — printing doctor blades, precision shims, appearance parts.
- Matte: Light texture finish, either from controlled atmosphere annealing or post-temper surface treatment. Good for applications needing paint adhesion or reduced glare.
When specifying, state your surface requirement explicitly. "Hardened and tempered" alone does not define the surface — and rework after delivery is expensive. See our full product range for available surface options by grade.
How to Specify Heat-Treated Strip
A complete specification to your supplier should include:
- Grade: e.g., SK5 (JIS), C75S (EN), 1075 (AISI)
- Delivery condition: Spheroidized annealed OR Hardened & tempered
- Hardness range: e.g., HV 480–540 (for H&T) or HV max 200 (for annealed)
- Dimensions: Thickness (±tolerance), width (±tolerance), coil ID/OD or cut length
- Surface finish: Blue, gold, polished, matte
- Edge condition: Slit edge, round edge, deburred
- Flatness / camber requirements: Critical for precision stamping
Incomplete specs lead to assumptions — and assumptions lead to rejected shipments. If you are unsure which condition suits your application, contact our technical team with your end-use description and we'll recommend the right combination.
Common Mistakes Buyers Make
- Ordering the wrong condition: Match annealed or hardened-and-tempered strip to the forming and heat-treatment route available for the finished part.
- Specifying hardness without temper color: Two strips at HV 500 can look completely different. If your customer expects blue-tempered finish, state it. Don't assume.
- Ignoring spheroidization condition: "Annealed" is not a single condition. Define the required microstructure and ask for a metallographic report where formability is critical.
- Choosing from price alone: Compare grade, processing route, testing and finished-part performance instead of assuming a fixed cost or scrap advantage.
- Not specifying edge condition: Slit edges on hardened strip can initiate fatigue cracks. If your application is cyclic, specify deburred or round edges.