1. Core Forming Technologies
The baseline geometric shape (typically elliptical or dished) of the cap is formed using one of the three primary methodsdepending on dimensions:
* Deep Drawing / Cold Stamping (Small to Medium Diameters): * This is the most common method for seamless caps of smaller sizes and standard wall thicknesses (e.g., SCH 40).
* Blanking: A hydraulic shear cuts a steel plate into a precise circular blank.
* Drawing: A hydraulic press forces the circular plate through a female die using a male punch. The metal stretches smoothly over the punch, taking on the required semi-spherical or elliptical shape.
* This can be performed at room temperature (cold drawing) or with
pre-heating (warm drawing), depending on material hardness and thickness.
* Hot Press Forming (Large Diameters & Thick Walls): * For heavy-walled or large-diameter caps (e.g., 18 inches and above), room-temperature stamping cannot deliver sufficient force to deform the metal.
* Heating: The steel blank is heated in an industrial furnace above its recrystallization temperature (typically 900°C–1100°C for carbon steel) to maximize its malleability.
* Molding: A heavy-duty hydraulic press swiftly stamps the glowing-hot steel plate into shape. This method reduces material stress and improves grain flow.
* Segmental Piece Welding (Extra-Large Custom Diameters): * For massive pipelines or pressure vessels spanning several meters in diameter, standard single-piece molds cannot be used.
* Petal Cutting: Individual curved sections, resembling orange peels or petals, are pressed separately.
* Assembly & Welding: These segments are aligned, tacked, and permanently joined using automatic or semi-automatic arc welding (e.g., Submerged Arc Welding - SAW), followed by full radiographic evaluation of the weld seams.
2. Step-by-Step Production Line
Beyond the initial forming stage, a factory must guide each cap through a series of finishing steps to achieve compliance with
international manufacturing specifications like ASME B16.9:
1. Trimming & Sizing: * Freshly stamped caps leave behind jagged, uneven edges at the rim (the skirt).
* The cap is secured onto a specialized trimming machine or heavy-duty lathe to cut away excess material,
leaving a uniform straight-face height and a completely flat base.
2. Beveling (Edge Preparation): * Because these are butt welding fittings, the open rim must be machined with a specific weld bevel.
* The standard profile is a 37.5° bevel with a 1.6mm flat root face (land). This profile is cut using dedicated pipe-fitting beveling machines to allow full-penetration welding when joined to a pipe in the field.
3. Heat Treatment: * Severe deformation—especially cold stamping—induces high residual stresses and alters the microcrystalline structure of the metal, making it brittle.
* Caps are loaded into heat-treating furnaces for Normalizing or Annealing (for carbon steels) or Solution Annealing (for stainless steels) to restore mechanical toughness and fatigue resistance.
4. Surface Treatment & Cleaning: * Carbon Steel: Caps undergo shot blasting to strip away high-temperature mill scale formed during heat treatment, followed by a coat of rust-preventative oil or black paint.
* Stainless Steel: Caps are treated with acid pickling and passivation to remove surface impurities and restore the
corrosion-resistant chromium-oxide layer. Sanitary-grade caps receive additional mechanical or electropolishing down to tight roughness targets (e.g., 32 Ra).
3. Inspection & Quality Control (QC)
Before leaving the factory floor, processed caps must pass a rigorous matrix of inspections to guarantee field safety under pressure:
* Dimensional Verification: Quality engineers use specialized calipers and gauges to measure the outside diameter, wall thickness (especially at the crown, which thins during stretching), ovality, bevel angle, and total height against strict standard tolerances.
* Non-Destructive Testing (NDT): For high-pressure lines or segment-welded caps, Radiographic Testing (RT) or Ultrasonic Testing (UT) is executed to guarantee zero internal cracks, voids, or weld imperfections.
* Product Marking / Stamping: A permanent laser or steel stamp is applied to the outside shell. This mark lists the manufacturer name, specification standard (ASME B16.9), material grade (e.g., WPB, 316L), size/schedule (e.g., 6" SCH 80), and a unique Heat Number tied directly to the raw material mill certificate for 100% traceability.