Which parts are forged, which are turned from bar and which are cold-headed from wire, why ferrules must be annealed, which steels adapters and fittings are made from, and what is different about making 6000 psi flanges.

Every part in the catalogue starts as a blank. How the blank is made determines the strength of the part, its price and even the batch size in which it makes sense to order it. Our works use three methods.

• Forging (hot die forging). A billet cut from bar is heated and forged in a press: the metal fills the die cavity and takes the shape of the future part. This is how elbows, tees, crosses, complex bodies, large hose fittings and flanges are made.
• Turning from bar. The part is machined from round or hexagon bar on an automatic lathe or a CNC lathe. This is how straight adapters, plugs, nipples and nuts are made.
• Cold heading from wire. Wire from a coil is cut into slugs and formed without heating in several blows on a multi-station machine. This is how ferrules and hose fittings up to 3/4″ are made.

After that the routes converge: machining, dimensional and thread inspection, zinc plating with passivation, final inspection and packing. The diagram shows the operations that the main groups of parts go through.

Straight adapters, reducers, plugs and nipples are turned from bar. The bar is fed into the machine automatically and the part is machined in one cycle: facing, outside diameters, the through bore, the cone or face for the seal, the thread, grooves and parting off. The second end is finished on the sub-spindle or after the part is re-chucked. Adapters with a hexagon are made from drawn hexagon bar, or the flats are milled on the same machine.

Elbows, tees, crosses and the bodies of adapters with a swivel nut are forged. The bar is cut into billets, heated to 1150–1200 °C and forged in a press. The excess metal squeezed out between the die halves is called flash; it is trimmed off in a separate die, and the scale is removed by shot blasting. The forging already has the shape of the body, and all that is left for the machine is each end: the face, the thread and the seat. The bores are drilled from different sides so that they meet inside the body, and the burrs where they intersect are always removed, otherwise metal particles end up in the hydraulic oil.

An elbow can also be machined from a solid block, and for small batches this is sometimes done. But most of the block then goes into chips, and the grain of the metal is cut through. In a forging the grain follows the contour of the body and runs round the corners, so it is stronger exactly where the loads are highest. Forging, however, needs a die, and a die pays for itself only over a batch. That is why forged items can have a minimum order quantity, while parts turned from bar can also be made in small batches.

Ferrules are made on multi-station cold-heading machines. The low-carbon steel wire comes in coils, drawn to size and with a phosphate lubricating coat that helps the metal flow in the dies. The machine straightens the wire, cuts off a slug and in several blows in successive dies forms a cup, pierces the bottom and finishes the outer and inner profile. The metal flows in the die without producing chips, so there is almost no scrap, and one machine turns out dozens of ferrules a minute.

Cold heading does have a drawback, though. When metal is deformed without heating, it strengthens: its hardness rises and its ductility falls. This is called work hardening. In a ferrule it is not acceptable, because during crimping the ferrule has to deform plastically and grip the hose tightly. A work-hardened ferrule may crack at the edge or along the serrations, especially in the cold, or crimp unevenly and hold the hose less well.

That is why ferrules are always annealed after heading: they are heated to 650–700 °C, held and cooled slowly. The metal recrystallises, the work hardening is removed and the ferrule becomes ductile again. From the outside, a ferrule that has not been annealed looks exactly like a proper one; only a hardness tester or a test crimp will show the difference, so it is worth checking this step with any supplier.

After the furnace the ferrules are checked for hardness, and a few from each batch are test-crimped on hose and inspected: there must be no cracks at the edge or along the serrations. Bore and wall thickness are checked on every batch, and only then do the ferrules go to plating.

Hose fittings up to 3/4″ (DN20) are made on the same cold-heading machines. The wire is headed into a stem blank with a collar and a hose tail. CNC machines then drill the bore, cut the thread, machine the cone or sealing face and turn the ring grooves on the tail that hold the hose. Heading gives the stem the same advantages as the ferrule: almost no scrap, and the grain of the metal follows the contour of the part.

Above 3/4″ heading no longer pays, because the forming force grows with the cross-section of the part and large, expensive machines are needed. That is why fittings of 1″ and above are forged or turned from bar.

In crimping, the stem and the ferrule have different jobs. The stem has to keep its shape and bore, while the ferrule has to grip the hose tightly, which is why mandatory annealing applies to ferrules.

Chinese works designate steel under the national GB/T standards, and those are the grades you will see in the certificates. The table lists the grades usually used for hydraulic connections and their nearest international equivalents. The steel grade for a particular batch is stated in the material certificate.

Works documents use several designation systems.

• 10, 20, 35, 45 are quality carbon steels to GB/T 699. The number is the average carbon content in hundredths of a per cent, so steel 45 contains about 0.45 % carbon. In works documents the number often carries a “#” sign: “45#” is the same steel 45.
• SWRCH and ML are cold-heading steels. SWRCH is the Japanese designation to JIS G3507, which Chinese steel mills use as well; ML is the Chinese designation to GB/T 6478, for example ML08Al.
• Grades beginning with the letter Y are free-cutting steels to GB/T 8731 with added sulphur or lead, which makes them easier to machine. Y15Pb, for example, is close to the American 12L14.
• 06Cr19Ni10 and 06Cr17Ni12Mo2 are stainless steels, the equivalents of AISI 304 and 316. Chinese documents often give them in the Japanese system: SUS304 and SUS316.

For every heat, the steel mill issues a certificate with the heat number, the chemical composition and the mechanical properties of the steel. The types of such documents are defined by the European standard EN 10204. In China the counterpart is GB/T 18253, which is identical to the international ISO 10474 and has the same document types with the same numbering. The first digit of the type shows what the results are based on: 2 means the manufacturer’s routine production control, 3 means tests on the very products being delivered. Two types matter most to a buyer.

There are also type 2.1, a declaration of compliance with the order without test results, and type 3.2, which is the same as 3.1 but with the results additionally confirmed by the purchaser’s inspector or an independent inspection body. If you need a 3.1 certificate for the material of a batch, please say so in your enquiry. Plating and salt-spray testing are covered in the article “Materials and finish”.

Code 62 flange fittings to SAE J518, also known as ISO 6162-2, are rated for 42 MPa (420 bar) in all sizes. Code 61 flanges have a lower rating, and it falls as the size increases. To hold 420 bar, the Code 62 flange head is made larger in diameter and thicker, and the bolts are spaced further apart. For example, on a 2″ fitting the Code 61 head is 71.4 mm in diameter and 9.53 mm thick, while the Code 62 head is 79.4 mm in diameter and 12.57 mm thick. That is why they are not interchangeable with Code 61 flanges.

The blank for a flange fitting is forged. The flange head is much wider than the tail, more than twice as wide on small sizes, and turning the part from bar the diameter of the head would send most of the metal into chips. More important still, in a forging the grain runs round the transition from the head to the tail. This is the most heavily loaded point of the part: the clamp halves press on the back of the head, while the pressure acts from inside with pulsations.

The strictest machining requirements apply to the sealing face and to the head. On the face the standard allows only circumferential tool marks with a roughness no coarser than Ra 3.2 µm, because a radial scratch on this surface becomes a leak path. The O-ring groove is machined exactly to size so that the ring is compressed by the specified amount. The head thickness is held to a narrow tolerance, otherwise the clamp halves will not press the head evenly against the port all the way round.

Flange fittings for 6000 psi come in three versions: for braided hose 1SN and 2SN, for spiral hose 4SH, and an interlock version for skived hose. In the interlock version the ferrule locks mechanically onto the stem when crimped, so the ferrule and stem of such a connection are made as a matched pair.

We work in Zhuji, in the town of Diankou in Zhejiang province. It is one of the largest metalworking centres in China: thousands of companies operate here, from large plants to small workshops making dies and tooling. For fitting production this means short supply lines, because bar, wire, dies and cutting tools are all within the same district.

• Division by product and by operation. In China production is usually divided up: the site that makes ferrules does not turn flanges, and a forge supplies forgings to several machine shops. Each site sets up its equipment, its people and its inspection for its own part, and this gives a price that is hard to achieve at a general-purpose plant. How this is organised at our works is shown on the Factory page.
• High machine utilisation. Automatics and CNC machines run round the clock, and one operator usually tends several machines. So large runs come out inexpensive, while in a small batch the changeover makes up a noticeable share of the price.
• A shared system of part numbers. Many Chinese works use the same fitting numbers: 20411, 26711, 87611 and so on. A number from one works’ catalogue is usually understood by another, and our catalogue keeps these numbers. The parts themselves are made to international standards — SAE, ISO, DIN, BS and JIS — while materials are designated to the Chinese GB/T standards.
• Different prices for the same item. The same catalogue item can differ noticeably in price from one works to another, and the difference almost always lies in what a photograph does not show: the steel grade, the annealing of ferrules, thread accuracy, zinc thickness and the type of passivation.

So when choosing a supplier, us included, it is worth asking a few questions.

1. What steel the parts are made of, and whether there is a certificate for it.
2. Whether ferrules are annealed after heading, and how their hardness is checked.
3. What the threads and seats are checked with, and how often.
4. What the zinc thickness is, which passivation is used and how many hours the coating lasts in salt spray before white corrosion.
5. How a batch is marked, and whether its test report can be traced from the marking.