Knowledge base
Knowledge base: special fastener terminology
What the terms really mean
For buyers and engineers
What the terms really mean
3.1 or 3.2 certificate, rolled after heat treatment, reduced-shank bolt to DIN 2510, property class 10.9, 8UN: anyone sourcing special fasteners comes across terms that determine price, lead time and safety. In this knowledge base, RS Randack explains the technical terms as they are actually used in production in Hagen: concisely, with figures, with references to the relevant production step and with a note on what you should specify in an enquiry. The ten most important terms come first, ranked by how often they appear in our customers’ enquiries; below them you will find all terms by topic.
Inspection certificates 3.1 and 3.2 to EN 10204
Which certificate you really need and how a witnessed inspection on our premises works.
Thread rolling or thread cutting
Two ways to make a thread, and why rolled threads have higher fatigue strength.
Rolling after heat treatment
Rolling threads after quenching and tempering: the gold standard for dynamically loaded joints.
American UN, UNC, UNF and UNR threads
Inch threads, stud bolts and material grades B7, L7 and B16.
Material verification and heat traceability
Why the heat remains traceable right through to the certificate.
How to use the wiki
On every page of this website, technical terms are marked with a dotted underline in the text. A click takes you to the explanation, and hovering the mouse over a term shows a short version. Every explanation ends with a section on what you should specify in an enquiry, so reading turns directly into a complete enquiry. If a term is missing, write to us and we will add it.
Wiki
All terms by topic
83 technical terms covering certification, materials, threads, products, production and surface treatment, explained from the shop floor in Hagen. On every page, technical terms in the text are clickable.
Fundamentals
- Fatigue strength and notch effectWhy bolts under cyclic loading fracture in the thread, and what helps prevent it
- Property class 10.9What 10.9 means, how it is achieved and how it is verified
- Nut property classWhich nut goes with which bolt, and what the proof load test demonstrates
- PreloadWhat really holds a bolted joint, and why torque is only an indirect measure
Certificates and third-party inspection
- Inspection bodyWho witnesses the tests and how an inspection visit works
- Inspection certificates 3.1 and 3.2Which certificate you really need for your bolts
- Stamp authorisationWhen quality assurance may stamp on behalf of the inspection body
- Batch and heatTwo numbers that trace every bolt back to the steel mill
- First article inspectionHow dimensions are released and documented at RS Randack
- Material tagHow bar stock is booked in and released at RS Randack
- Test barWhy a companion specimen from the same heat accompanies every lot
- Material verification and batch traceabilityWhy the heat remains traceable right through to the certificate
- Declaration of compliance with the order 2.1 and test report 2.2The basic inspection documents to EN 10204
- Works orderThe document that accompanies every lot through production
Materials testing
- Dye penetrant testingCrack testing for stainless steels, duplex, nickel-based alloys and titanium
- Metallographic examinationWhat the microscope reveals about heat treatment, forging and coating
- Hardness testingThe quick check of the quenched and tempered condition for each batch
- Calibrated test equipmentWhat RS Randack uses to measure dimensions, threads and surfaces
- Charpy impact testWhat impact energy tells you about toughness at low temperatures
- Magnetic particle testingRevealing surface cracks in magnetisable steels
- Inspectors certified to ISO 9712Who is authorised to test at RS Randack and what the levels mean
- Crack testingHow surface cracks in bolts are made visible
- Spectrometric analysisHow the chemical composition of a steel is checked in-house
- Ultrasonic testingFinding internal defects in bar stock and thick cross-sections
- Tensile testTensile strength, yield strength and elongation on test bars and full-size bolts
Approvals and codes
- AD 2000 W0What assessment to AD 2000 W0 means for bolts on pressure equipment
- BV Mode IIWhat Bureau Veritas approval means for bolts, nuts and turned parts on board
- DNV approvalWhat an Approval of Manufacturer from the classification society DNV means for your order
- Pressure Equipment DirectiveWhy flange bolts fall under the Pressure Equipment Directive and what the manufacturer must demonstrate
- EN 15048 and CE markingWhen a bolted connection in construction requires CE marking
- ISO 9001 and ISO 14001What the two certificates state and why they are the basis for all other approvals
- KTA 1401What the KTA 1401 qualification certificate means for fastener suppliers
Materials
- ASTM A193 B7, B16 and B8The American material grades for pressure equipment and flanges
- ASTM A320 L7Bolts with Charpy impact verification at –101 °C
- Duplex steelTwice as strong as austenitic steel, resistant to chlorides
- European raw materialWhy the origin of the steel matters to buyers
- Stainless steelsCorrosion-resistant, non-magnetic and demanding to process
- Nickel-based alloysWhen steel is no longer enough: heat, chemicals and cryogenic temperatures
- TitaniumHalf the weight of steel at the same strength
- Quenched and tempered steelWhich steel achieves which strength at which diameter
- Creep-resistant steelsMaterials for bolts that retain their strength at 300 to 600 °C
- Material number1.7225, 42CrMo4, B7, UNS: how a steel is uniquely identified
Threads
- American UN, UNC, UNR threadsUnderstanding inch threads, stud bolts and material grades B7, L7 and B16
- Through-feed and in-feed rollingTwo ways to roll a thread, and when each is chosen
- Pitch diameter and thread gaugesThe dimension that determines the fit, and how it is checked
- Thread rolling or thread cuttingTwo ways to make a thread, and when each is the right one
- Thread tolerances 6g and 6HWhat the tolerance class tells you about fit, coating and inspection
- ISO 6157 surface discontinuitiesWhich discontinuities are permissible, which are not, and how they are inspected
- UNJ and MJ aerospace threadsWhy the large root radius is decisive for fatigue strength
- Coarse and fine threadsWhen a fine pitch is the better choice and what it demands in production
- Rolling after heat treatment (final rolling)Rolling threads after quenching and tempering
Products
- Extension sleeveWhy a sleeve under the nut protects the joint against embedding and temperature effects
- Reduced-shank bolts to DIN 2510Why the slim shank makes the joint more reliable
- Threaded rods and anchor boltsLong rolled threads for foundations, tunnels, railways and plant engineering
- T-head boltThe head that hooks in from behind: for T-slots, crane rails and steel construction
- Heavy hex nutThe heavy hexagon nut for stud bolts and flanged joints
- Fitted boltWhen the bolt must not only clamp but also centre and transmit shear forces
- Studs & boltsWhat customers mean when they ask for studs and bolts, and how RS Randack supplies them
- Hex boltFrom standard parts in special sizes to forged bolts made to drawing
- Special nuts and cap nutsNuts to drawing from our own nut machining, typically up to M80
- StudThe most requested product at RS Randack, and what matters at the metal end
- Stud boltFully threaded, two heavy hex nuts, material grade to ASTM
- Socket head cap screwWhen the head has to sit recessed and the strength still has to be right
Production
- Drop forging and grain flowWhy a forged bolt head withstands more than a turned one
- Induction heatingHow a bar section reaches forging temperature in seconds, and what that means for the part
- MarkingManufacturer’s mark, property class, material, batch: what is marked on the part and why
- Steady restHow shafts, tie rods and bolts up to 3,500 mm long are turned to size
- StraighteningWhy long bolts are checked after heat treatment and straightened if necessary
- GrindingWhen a turned part needs to be ground and what this achieves
- Die-sinking EDMProducing shapes in hardened steel without cutting
- Packing and corrosion protectionHow bolts and nuts arrive in the condition in which they left the works
Heat treatment and surface treatment
- Hot-dip galvanisingThick zinc coating for steel construction, outdoor use and infrastructure, with oversize-tapped nuts
- Zinc electroplatingThe classic coating up to 8.8, and why it needs approval for 10.9
- Anti-friction coating and coefficient of frictionWhy the coating determines how much of the torque becomes preload
- Solution annealingThe heat treatment for stainless steels and nickel-based alloys
- Manganese phosphateThe base layer for high pressure, anti-friction coatings and oiled storage
- Neutral salt spray testWhat 480, 720 or 1,000 hours tell you about a coating, and what they do not
- Coating thicknessHow coating thickness is measured and why, on threads, it determines whether the parts can be assembled
- Stress-relief annealingThree annealing processes, what they do and when they make sense for bolts and turned parts
- Baking (hydrogen de-embrittlement)Why high-strength bolts are heated after zinc plating, and what this can and cannot do
- Quenching and temperingHow Q&T steel achieves its property class and how it is verified
- Hydrogen embrittlementCause, risk and how it is avoided
- Zinc flake coatingCorrosion protection for high-strength bolts without hydrogen embrittlement
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