BK+S is a delivery condition for cold-drawn precision tubes; it is not automatically a polished or finished bore. Copy the approved grade, finishing route, bore tolerance, roughness, dimensions, and inspection-document type from the drawing. The Kanpur quotation must confirm the exact offered size and condition; this page provides no stock band.
What does BK+S mean on a precision-tube quotation?
BK+S means cold drawn and stress relieved after drawing. The tube keeps tight dimensions and useful strength, but residual stresses are reduced. It is a delivery condition, not a complete surface-finish specification unless the RFQ also states honing, skiving, or roughness.
In older DIN 2391 language, BK indicates hard as drawn and BK+S indicates stress relieved after drawing. EN 10305 uses related condition language, and suppliers often keep both vocabularies on quotations because Indian buyers recognise BK+S. The important point is that BK+S describes heat treatment after cold work. It does not by itself guarantee an H8 bore or Ra 0.4 micrometre finish.
Cold drawing can leave residual stress, and a stress-relief operation changes the delivery condition. The responsible process plan should evaluate movement during cutting, boring, welding, honing, and end machining rather than assuming a condition is suitable from the component name.
A complete RFQ should state the approved standard, grade, delivery condition, dimensions, finished-ID tolerance, surface-texture limit, inspection document, and any forming or welding steps. Do not select BK+S, NBK, H8, or another condition from a generic barrel, spacer, bushing, bending, or flaring rule.
How does BK+S affect hydraulic seal life?
BK+S is a delivery condition, not a seal-life rating. The approved cylinder and seal design must control bore size and geometry, surface texture, cleanliness, material, pressure, temperature, fluid, lubrication, and manufacturing route.
Hydraulic seals fail when the bore is too rough, too loose, too tight, out of round, or contaminated. BK+S helps by giving the finisher a stable tube. A stable tube is easier to hone consistently, easier to hold straight, and less likely to distort after end machining. However, the seal does not ride on the delivery-condition label; it rides on the actual bore surface.
Specify the final bore with the exact tolerance and surface-texture parameters required by the drawing and seal supplier. Do not assign H8, H9, an Ra band, or another texture parameter from a generic duty description; state the measurement method and acceptance report as well.
Cleanliness is the forgotten part of seal life. Honing residue, rust-preventive sludge, or grinding dust can score seals during first movement. Ask how the bore is cleaned and protected after finishing. Capping and oiling may look small, but they protect the most expensive feature of the tube.
When should a buyer choose BK+S instead of BK, NBK, or GBK?
Choose BK+S, BK, NBK, GBK, or another delivery condition only from the approved material and process plan. The component name, machining, welding, forming, or hydraulic use does not select a condition by itself.
Verify each condition against the applicable standard and material data. Do not infer maximum strength, tight dimensions, residual-stress response, ductility, softness, stability, or application from the shorthand alone.
The approved material and process plan should account for welding, flaring, bending, forming, machining, heat treatment, inspection, and service. These delivery conditions are not generic quality levels or application presets.
Procurement should record every operation after supply and copy the approved condition into the RFQ. Any proposed substitution needs a property and process comparison plus design approval; do not justify it with assumed scrap or cost savings.
What inspection records prove a BK+S tube is acceptable?
The MTC should show grade, heat number, mechanical properties, and delivery condition. For hydraulic barrels, add dimensional readings, bore tolerance, straightness, and surface-roughness report. Visual inspection should check dents, rust, ID protection, and cut-end damage before machining starts.
The purchase order should name the required EN 10204 inspection-document type. That document does not by itself prove a finished bore meets its dimensional or surface requirements, so request the specified final reports when honing or skiving is included.
A useful inspection report lists OD, ID, wall, length, straightness, bore tolerance, and roughness. The roughness entry should use ISO 4287 terms, not “good finish.” If the application is high value, specify the sampling plan and measurement locations along the tube length.
Incoming inspection should protect the bore. Do not drag probes, chains, or forks through the ID. Check for dents at the mouth, rust staining, water inside the bore, and missing caps. A minor-looking dent can destroy a piston seal or force extra machining allowance.
Worked weight calculations for BK+S E355 cylinder tubes
BK+S does not change the nominal steel-weight method. Use OD and wall for a planning estimate before honing or skiving, then reconcile the result with the ordered length, measured dimensions, MTC, and dispatch scale weight.
For an 89.0 mm OD E355 BK+S tube with a 5.0 mm wall, the unrounded site formula is (89.0 − 5.0) × 5.0 × 0.02466 = 10.3572 kg per metre, displayed as 10.36 kg/m. At the displayed rate, a 6-metre length is 62.16 kg and 80 such lengths are 4,972.80 kg, or about 4.97 MT. These figures plan material and handling; they do not replace the actual dispatch weight.
The 89.0 mm OD row in the site's indicative DIN 2391 price list is ₹95 to ₹118 per kg. Apply that band only to the matching size when preparing a budget. BK+S condition, final bore finish, tolerance reports, quantity, and delivery destination can affect the firm quotation, so the PO should identify each requirement separately.
Stress relief reduces residual stress from cold drawing and can improve stability during machining, but it does not guarantee straightness or finished-bore acceptance. Long-stroke barrel buyers should state the required straightness and measurement method on the drawing. If machining opens the bore or adds ports, inspect the finished component before assembly rather than relying on the delivery-condition label.
Procurement and delivery of BK+S precision tubes in Kanpur and Lucknow
A BK+S precision-tube quotation should state the offered size and condition, material location, inspection documents, legally applicable GST rate, freight basis, and delivery timing. Do not infer stock, inspection access, or dispatch capability from this page.
The business address is 83/124, Steel Market, Bartan Bazar, Afim Kothi, Basanti Nagar, Juhi, Kanpur 208014. Buyers can request an ex-warehouse Kanpur comparison basis, but the current written quotation must confirm the offered lot's location, stock status, pickup option, and destination-freight terms.
PWD, irrigation, JJM, and industrial buyers should state the required certificate in the tender enquiry. An EN 10204 3.1 MTC identifies the heat and reports chemical and mechanical properties, including the values used to verify E355. The buyer should also request a dimensional or bore report when the finished tube must meet H8, H9, straightness, or roughness requirements.
Provide the delivery location and required date with the RFQ. Require the quotation to confirm stock, freight, dispatch terms, and whether any requested TPI can be coordinated at the material location; do not apply a general service level or assume a Kanpur-yard inspection.
How to specify BK+S precision tubes on your RFQ
Treat BK+S as the delivery condition only. Add finished bore class, roughness, length, MTC, and the Kanpur commercial basis as separate lines so a stress-relieved tube is not quoted as a honed barrel by default.
Write the approved standard, grade, and delivery condition on the first line. Then state OD and wall or OD and ID, cut length, quantity, and the approved finishing requirements. Do not assign BK+S, H8, H9, honing, or skiving from a hydraulic-barrel, spacer, or bushing label.
Ask the quotation to state the offered lot's location, whether inspection is available, and the inspection-document type required by the PO. Add a dimensional or roughness report when the supplier is also finishing the bore. Keep the legally applicable GST rate on the current quotation, packing, and freight as separate lines.
Do not substitute BK, NBK, or GBK for BK+S without drawing approval. The condition changes residual stress, welding behaviour, and machining movement. Destination, required date, and whether yard pickup or arranged freight is needed should appear on the RFQ rather than in a later phone call.
Keep technical acceptance separate from commercial comparison. A bid tab should reproduce the ordered standard edition, grade, condition, dimensions, tolerance limits, manufacturing route, finishing scope, inspection documents, and permitted substitutions before showing price or delivery. Mark any blank supplier response as unresolved instead of assuming it matches the requested condition.
At receipt, reconcile the purchase order, quotation, packing list, physical marking, and inspection document before cutting or machining. Record the offered condition exactly as certified, check the dimensions and surface requirements named by the inspection plan, and route any mismatch for disposition. This preserves an auditable boundary between the delivered parent tube and later buyer-performed finishing.
- Give OD and wall in millimetres, not a nominal inch size.
- Name the approved standard, grade, and delivery condition.
- Add the approved finished-bore limits and surface-texture parameters where required.
- Require the inspection-document type named by the purchase order, plus a bore report if honing or skiving is included.
- Quote ex-warehouse Kanpur, then state pickup or destination freight after stock is confirmed.