I. Sampling Locations (Critical! Sampling positions vary significantly by product form)
Sampling location directly affects test results; therefore, strict provisions apply to different material forms in relevant standards.
1. Steel Plates (Most Common)
In accordance with GB/T 2975 Steel and steel products — Location and preparation of test pieces for mechanical property testing
(1) Tensile Specimens
- Rolling direction: Transverse (perpendicular to rolling direction).
- Thickness direction:
- Thickness ≤ 25 mm: Full-thickness sampling (full-thickness plate specimen).
- Thickness > 25 mm: Sampling at 1/4 thickness (due to different properties between surface and core).
(2) Charpy Impact Specimens
- Rolling direction: Transverse.
- Thickness direction:
- Thickness ≥ 12 mm: Sampling at 1/2 thickness (best represents overall toughness of the material).
- Thickness 6–12 mm: Full-thickness specimen with notch machined on the surface.
- Thickness < 6 mm: Impact test not required per standard.
(3) Bend Specimens
- Rolling direction: Transverse.
- Sampling position: Same as tensile specimens.
2. Steel Pipes (Seamless and Welded Pipes)
In accordance with GB/T 2975 and GB/T 6396
(1) Tensile Specimens
- Sampling direction: Transverse (perpendicular to pipe axis).
- Sampling position:
- Seamless pipes: Sampled at 1/2 wall thickness of the pipe body.
- Welded pipes: Sampled in base metal area, avoiding welds.
(2) Charpy Impact Specimens
- Sampling direction: Transverse.
- Sampling position:
- Seamless pipes: At 1/2 wall thickness.
- Welded pipes: Normally sampled in the weld heat-affected zone (HAZ) if required by standards or technical agreements.
3. Forgings
In accordance with GB/T 2975 and NB/T 47009
- Tensile specimens: Sampled at 1/4 radius below the forging surface or as specified on drawings.
- Impact specimens: Sampled at the same position as tensile specimens.
- Large forgings: Sampling required at multiple locations (surface, 1/2 radius, center, etc.).
4. Welded Joints (Critical for Pressure Vessels)
In accordance with NB/T 47014 and GB/T 2649
- Tensile specimens: Along the central axis of the weld.
- Impact specimens: Notches machined separately in weld metal, heat-affected zone (HAZ), and base metal.
- Bend specimens: Face bend, root bend, and side bend.
II. Specimen Preparation (Standard-Compliant Practice)
Specimen preparation must ensure:
- No alteration of material properties
- Accurate dimensions
- Qualified surface quality
1. Sampling Methods
- Mechanical cutting: Sawing, milling (most commonly used).
- Flame cutting: Only permitted for rough cutting with sufficient machining allowance (generally ≥ 10 mm) to avoid thermal influence.
- Prohibited: Direct oxyacetylene cutting to final dimensions.
2. Machining Requirements
- Surface roughness: Generally Ra ≤ 1.6 μm.
- Parallelism and perpendicularity: Must meet tolerance requirements of GB/T 228 and GB/T 229.
- Transition radii: Smooth fillets required at shoulders of tensile specimens to avoid stress concentration.
- Stress relief: Avoid cold work hardening after machining (annealing if necessary).
3. Specimen Types
(1) Tensile Specimens
- Proportional specimens: e.g., φ10 mm round bar specimen (L₀ = 5d).
- Plate specimens: For thin and thick plates.
- Full-thickness specimens: For plate thickness ≤ 25 mm.
(2) Charpy Impact Specimens
- Standard specimen: 10×10×55 mm (V-notch).
- Sub-sized specimens: 7.5×10×55 mm, 5×10×55 mm (used when thickness is insufficient).
(3) Bend Specimens
- Dimensions: Width b = 20 mm (or as per standard), length L = 150–200 mm.
- Edges: Must be chamfered to prevent cracking during bending.
III. Summary (Engineers’ Common Memory Rule)
- Steel plates: Tensile — transverse at 1/4 thickness; Impact — transverse at 1/2 thickness.
- Steel pipes: Transverse sampling at 1/2 wall thickness.
- Forgings: At 1/4 radius.
- Welded joints: Sampling required from weld, HAZ, and base metal.
- Specimen preparation: No thermal influence, accurate dimensions, smooth surface.
