HRB335 vs HRB400 – Composition, Heat Treatment, Properties, and Applications

Introduction

HRB335 and HRB400 are two widely used grades of hot‑rolled deformed reinforcing bars (rebars) commonly specified in structural concrete work and many fabrication contexts. Engineers, procurement managers, and manufacturing planners frequently weigh the tradeoffs between lower‑cost, more ductile reinforcement versus higher‑strength materials that permit smaller sections or longer spans. Typical decision contexts include balancing cost against structural weight, selecting rebar for seismic detailing where ductility is paramount, and choosing a grade compatible with welding, bending, or fabrication processes.

The defining difference between these two grades is their yield strength level: HRB400 is specified at a higher nominal yield than HRB335. Because yield strength strongly influences section sizing, reinforcement detail, and forming/welding behavior, HRB335 and HRB400 are commonly compared in design and procurement discussions.

1. Standards and Designations

• GB/T 1499.2 (China): expressly defines hot‑rolled ribbed steel bars; HRB335 and HRB400 are Chinese designations.
• ASTM A615 / ASTM A615M (United States): specification for deformed and plain carbon‑steel bars for concrete reinforcement (uses grade numbers differently).
• EN 10080 / EN 1992 and national annexes (European practice): general standards for reinforcement; European designations use B500 or similar grade numbers.
• JIS G3112 (Japan): specification for deformed steel bars for concrete.
• ISO standards and national deviations also exist for bar tolerances and testing.

Classification: Both HRB335 and HRB400 are carbon‑manganese (C–Mn) reinforcing steels, sometimes produced with microalloying additions (V, Nb, Ti) or controlled rolling routes; they are not stainless, tool, or high‑alloy steels. They belong to the mild/medium‑carbon structural steel family used for reinforcement rather than for high‑temperature or wear applications.

2. Chemical Composition and Alloying Strategy

Element	HRB335 (typical practice)	HRB400 (typical practice)
C (carbon)	Controlled as the primary hardenability/strength influencer; kept relatively low for ductility	Controlled; may be slightly higher or balanced by other alloying/hardening methods to achieve higher yield
Mn (manganese)	Principal strength and deoxidation element; present at controlled levels	Present at controlled or slightly elevated levels to increase strength and hardenability
Si (silicon)	Minor deoxidizer; usually low	Minor deoxidizer; similar to HRB335
P (phosphorus)	Limited impurity; kept low for toughness	Limited impurity; kept low for toughness
S (sulfur)	Limited impurity; kept low to improve ductility and weldability	Limited impurity; kept low
Cr, Ni, Mo	Typically not intentionally added in significant amounts for standard rebar	Typically not intentionally added for standard rebar (may appear in trace amounts)
V, Nb, Ti	May be present in microalloyed rebars for grain refinement and strength	More commonly used in thermo‑mechanically treated or microalloyed HRB400 to raise yield without much carbon increase
B	Not generally used in rebars	Not generally used
N (nitrogen)	Controlled as impurity/interstitial	Controlled as impurity/interstitial

Notes: - Rebar mills achieve higher nominal yield in HRB400 either by modest increases in alloying/hardenability or, more commonly, by thermo‑mechanical controlled rolling and accelerated cooling plus microalloying (Nb, V, Ti) to refine grain size and raise yield while keeping carbon low to preserve weldability. - Exact chemical limits are defined in relevant standards and by manufacturers; composition varies by mill practice and by whether the product is “ordinary” HRB or microalloyed/thermo‑mechanically processed.

How the alloying strategy affects behavior: - Carbon and manganese principally control base strength and hardenability. - Microalloying with Nb, V, Ti promotes precipitation strengthening and grain refinement, enabling higher yield without significantly increasing carbon. - Low levels of alloying are intentionally maintained to preserve ductility and weldability typical for reinforcement steels.

3. Microstructure and Heat Treatment Response

Typical as‑rolled microstructures for rebars are ferrite plus pearlite (ferrite–pearlite). Differences arise from processing:

• HRB335 (conventional hot rolling): generally shows a relatively coarse ferrite–pearlite microstructure with good ductility. If manufactured by basic hot rolling with air cooling, the microstructure remains largely ferritic with pearlitic islands.
• HRB400 (higher strength): often produced by controlled rolling and controlled cooling (thermo‑mechanical processing). This yields finer ferrite grain size, more uniformly dispersed pearlite, and in some processes a partially bainitic microstructure where accelerated cooling is used. Microalloy precipitates (NbC, V(C,N), TiC) further refine grains and raise yield.

Heat treatment response: - Normalizing or quenching & tempering are not typical for standard grade rebars due to cost and the impracticality for long sections; however, localized heat (welding) can affect microstructure in the heat‑affected zone (HAZ). - Thermo‑mechanical controlled processing (TMCP) can produce HRB400‑equivalent properties without post‑rolling heat treatment by manipulating rolling temperature and cooling rate. - Quenching & tempering is a pathway to higher strength grades but is more common for bar steels used in engineering components rather than standard reinforcement.

4. Mechanical Properties

Property	HRB335	HRB400
Yield Strength (nominal)	335 MPa (designation basis)	400 MPa (designation basis)
Tensile Strength	Typical moderate tensile strength suitable for concrete reinforcement	Higher tensile strength than HRB335; suited to reduced section design
Elongation (ductility)	Generally higher elongation / greater ductility	Typically lower elongation than HRB335 but still required to meet ductility limits in codes
Impact Toughness	Good toughness at ambient temperatures when manufactured to standard practice	Can match toughness if produced with controlled rolling; may be more sensitive to processing
Hardness	Lower than HRB400 in comparable mill practice	Higher hardness corresponding to higher yield; may be more susceptible to brittle fracture if not properly processed

Explanation: - The yield level is the primary distinguishing mechanical parameter. HRB400 provides a higher yield plateau enabling smaller reinforcement areas for the same design strength. - Ductility and elongation requirements are specified in standards; exceeding minimum ductility is critical for seismic detailing. Because HRB335 typically reaches required ductility more easily, it can be preferred where capacity for plastic deformation is prioritized. - Impact toughness depends more on production route than grade name; modern HRB400 produced by TMCP with microalloying can achieve acceptable toughness.

5. Weldability

Weldability of reinforcement is governed by carbon equivalent and hardenability; lower carbon and lower hardenability improve weldability.

Useful index examples: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation: - HRB335, with its generally lower hardenability and lower or comparable carbon equivalent, is typically easier to weld using conventional SMAW, GMAW, or FCAW procedures with standard preheat and low‑hydrogen practices. - HRB400, especially if microalloyed or thermally processed, can have higher hardenability; care with preheat, interpass temperature, and post‑weld heat treatment (if required by design) may be necessary to avoid HAZ cracking. Nevertheless, many HRB400 products are formulated to be readily weldable for field splicing and fabrication. - For critical welded connections, perform joint qualification and pre/post‑weld procedures per welding codes; use CE or Pcm calculations to estimate susceptibility to cold cracking.

6. Corrosion and Surface Protection

• HRB335 and HRB400 are non‑stainless carbon steels; they rely on concrete cover for corrosion protection in reinforced concrete and on coatings when exposed.
• Common protections: hot‑dip galvanizing (zinc coating), epoxy coatings, mechanical surface treatments, and adequate concrete cover design combined with corrosion inhibitors.
• PREN (Pitting Resistance Equivalent Number) applies to stainless alloys for comparing localized corrosion resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not applicable to plain carbon rebars like HRB335/HRB400.
• Selection guidance: for aggressive environments (chloride exposure, marine), specify epoxy‑coated or galvanized rebars, or consider stainless grades when longevity outweighs material cost.

7. Fabrication, Machinability, and Formability

• Cutting: Both grades are typically cut by abrasive saws, flame cutting, or mechanical shearing. HRB400's higher strength may increase cutting forces and tool wear slightly.
• Forming and bending: Higher yield requires greater bending force. Bending radii and cold‑bending procedures are specified in codes; HRB400 will generally demand larger bending equipment and may have tighter limits on re‑bending than HRB335.
• Machinability: Reinforcing bars are not optimized for machining; both grades have similar poor machinability relative to low‑carbon free‑cutting steels. Use appropriate tooling and speeds.
• Finishing: Threading or mechanical couplers are widely used. HRB400 can be used with couplers designed for higher load levels; ensure compatibility with heat treatment of couplers and bar material.

8. Typical Applications

HRB335 (Typical Uses)	HRB400 (Typical Uses)
General reinforced concrete in buildings and infrastructure where ductility and economy are priorities	Structures that require higher load capacity per bar, longer spans, or reduced reinforcement area
Seismic detailing where high ductility and larger plastic rotation capacity are critical (subject to code limits)	Bridges, heavy foundations, and elements where higher yield enables thinner sections or fewer bars
Non‑critical fabrication and mass concrete where cost sensitivity is a priority	Prestressed anchorage zones, heavily loaded members, and retrofits where increasing strength without enlarging cross‑section is desired
Repair work using standard bar sizes and conventional couplers	Situations that permit more precise quality control and where higher strength rebar is specified

Selection rationale: - Choose HRB335 when ductility, ease of handling, and cost are prioritized—especially in seismic regions where plastic deformation capacity matters. - Choose HRB400 when design requires higher yield to reduce reinforcement quantities, achieve slimmer profiles, or meet specific structural load demands.

9. Cost and Availability

• Relative cost: HRB335 is generally less expensive per ton than HRB400 because of lower processing requirements and lower alloying/processing intensity. HRB400 may command a premium depending on production route (TMCP, microalloying) and market supply.
• Availability: Both grades are widely produced and available in standard product forms (coiled, straight lengths, cut‑to‑length) from major mills. Availability by diameter and form can vary by region; procurement should confirm mill test reports and delivery lead times.
• Product forms: plain bars, deformed bars, welded mats, coils—specify grade and production route in purchasing documents to avoid misinterpretation.

10. Summary and Recommendation

Criterion	HRB335	HRB400
Weldability	Very good (lower CE)	Good to moderate; may require controls depending on processing
Strength–Toughness balance	Good ductility; lower yield	Higher yield; can have lower ductility unless TMCP/microalloyed carefully
Cost	Lower	Higher (premium for higher yield/process)

Choose HRB335 if: - You require higher ductility and plastic deformation capacity for seismic detailing or energy‑dissipating elements. - Project cost sensitivity and ease of fabrication/welding are prioritized. - Standard reinforcement layouts and larger bar areas are acceptable to meet capacity.

Choose HRB400 if: - You need higher yield strength to reduce reinforcement area, slim down sections, or meet increased load demands without changing member geometry. - The production route (TMCP or microalloying) ensures adequate toughness and weldability for the intended application. - Project constraints favor material substitution to save space, weight, or to meet specific structural performance targets.

Final note: The numeric designations (335 and 400) indicate nominal yield levels, but performance in service depends on mill practice, processing history, and quality control. Always specify material standard, required mechanical properties, delivery condition, and testing/traceability when procuring rebars; request mill test certificates and, for critical applications, joint qualification for welding and bending.