Forming and Welding the Tube

At Penflex, metal hose is manufactured from stainless steel strip. Continuous coils are formed into tube and the longitudinal seam is welded. The weld is a full-penetration butt weld. The weld is TIG welded. "TIG" stands for "Tungsten Inert Gas". The tungsten is the electrode which means it is the conductor being used to carry the current (which supplies the heat for welding). The tungsten is not consumed during the welding process, so no material from the tungsten is transferred to the tube. All of the material which melts into the weld joint comes from the strip itself. No filler material is added to the longitudinal weld. The inert gas used in the welding process is argon. Inert means "non-reactive". When the high heat from the weld current causes the two edges of the joint to melt in the presence of oxygen, this oxygen reacts with the elements in the metal to form oxides. The effect of this oxidation is an area which may be more susceptible to corrosion than the adjacent base metal. For this reason, argon is introduced into the area right where the electrode is. This is called "purging" the weld. The argon flow pushes all of the oxygen out of the way so that it can’t react with the metal. Since the argon is an "inert gas", it does not have any effect. At Penflex, the inside of the tube is "completely purged" as argon is introduced and accumulated in the enclosed volume of the tube. Likewise, argon is flowing in the area of the tungsten on the outside of the tube, but because this area is open to the atmosphere, the argon flow must be constant; it doesn’t accumulate.

The Transformation from Tube to Hose: Mechanical Corrugation Method

The base metal thickness used in corrugated hose sizes from 1/4" to 2" varies from .008" to .024" depending on the need to resist pressure or corrosion. The ratio of the metal thickness to the diameter of the tube is relatively large, so the tube is somewhat stiff in terms of deflection and strong in terms of resistance against crushing. For this reason, the hose sizes from 1/4" to 2" can be formed by "scoring from the outside": When the tube is fed into the corrugator, it goes through a "cut ring" which rolls around it, pushing into the tube. The result is a "score" or a slight "neck-down" of the tube at uniform intervals as each section advances. These "neck-down" areas will ultimately become the "valleys" of the corrugations. The tooling is designed to have a smooth radius to minimize stress concentrations. Next, a pair of "C-shaped" tools on the corrugating machine called "jaws" clamp onto the tube right on the "scored" section. Then a second set of jaws clamps onto the adjacent "necked-down" section. One of these sets of jaws is stationary and the other set moves towards the stationary one. This causes the tube material between the jaws to "buckle up" into a curved shape to form the corrugations. The spacing between the "cuts" determines the length of material available to form the corrugation: The longer the space between "cuts", the "taller" the corrugation. The height of the corrugation relative to the ID of the hose is sometimes referred to as the "profile" of the hose. That is, we say that one hose has a "higher profile" than another if the difference between its OD and ID is greater than the same difference on the other hose.

The Transformation from Tube to Hose: Hydraulically-formed Method

As the tube size gets larger and larger, the thickness of the tube "wall" gets very thin in proportion to the diameter. The tube is not very "stiff" compared to the smaller sizes. In fact, the tube could be manually squeezed with little force and made into an oval shape. If you tried to use the "cut ring" process on this tube to form hose, the walls of the tube would "buckle-in" or collapse at the cut stage. For this reason, the larger size hoses are formed instead by pushing the metal “outward” from the tube rather than inward. This can be accomplished in several ways. At Penflex, the tube is slipped over a “bumping rubber” shaped something like a donut which fits just inside the tube. A hydraulic plunger squeezes the donut. As it flattens out, it also widens outward on its diameter and pushes into the wall of the tube, drawing the material from the tube up into a "bulge".

When the pressure on the plunger is removed, the elastic nature of the "bumping rubber" allows it to go back to its original shape. The tube advances a certain distance and then the process is repeated. At Penflex, each individual corrugation is formed one at a time. We refer to this method as "Hydraulically-formed" because a hydraulic plunger is used to create the force to push the material outward to form the corrugations. "Hydraulically formed" is not to be confused with another method of metal forming called "hydro-forming". In hydro-forming, the force used to push the metal outward to form the corrugation is generated by water instead of a compressed rubber donut. The water pressure from inside the tube pushes the metal into a "die" on the outside of the tube which gives the "bump" its shape. Hydro-forming may be used to form the corrugations individually, or in a group of several "humps" all at once in a "multi-station" form. Usually, the hydro-forming method is used on thinner walled tube and the thicker wall hoses are formed using the "bumping rubber" hydraulic method. At Penflex, all hoses sizes 2-1/2" and above are made using the "bumping rubber" hydraulic method. After the bumps are put in the tube, the stationary and moveable jaws grab the tube in the "valleys" between the corrugations. The moveable jaws press towards the stationary ones to form the corrugations. This step in the process is similar to that used in the mechanical corrugation method.

At first glance, it appears that 304L is very similar to 321. In fact, looking at the chemical composition, it is clear that the chromium and nickel ranges of the two alloys are very similar. The difference appears when the issue of “carbide precipitation” in the “heat-affected zone (HAZ)” is discussed. This is when the heat of welding causes the available chromium in the grain boundaries to combine with the carbon in the steel and this reduces the corrosion resistance in this zone. One of the ways to combat this phenomenon is to lower the carbon content (thus the "L" in 304L: .030% max vs. .080%) But an even more effective way is to add titanium to the alloy to “stabilize it”. The carbon likes to attach to the titanium better than the chromium, so it leaves the chromium alone so it is still available to resist corrosion. The 321 is an alloy with added titanium (it has to have at least 5 times the carbon percent). The reduced risk of corrosion in the HAZ is the main advantage of 321.

In dynamic applications, fatigue strength is also important. Fatigue or endurance limits (strength in bending) of austenitic stainless steels in the annealed condition are about one-half the tensile strength. Typical tensile and endurance limits for these alloys (annealed) are:

TEMPERATURE FACTORS - The factors are slightly higher for 321 than for 304L at most elevated temperatures, which for some applications can be a defining factor in choosing 321 over 304L.

(The design guide published by the Stainless Steel Producer’s of North America was used for researching this answer.)

The materials used to manufacture Penflex hose and braid do not exceed the tolerated concentration limits of hazardous substances subject to the RoHS Directive. Components and products supplied by Penflex do not contain more than the permitted levels of the following restricted substances:

• Lead
• Mercury
• Cadmium
• Hexavalent chromium
• Poly Brominated Biphenyles (PBBs)
• Poly Brominated Diphenyles (PBDEs)

(The maximum permitted amount by weight is .01% for cadmium and 0.1% for the remaining substances)

The maximum pressures of braided assemblies are based for the most part on theoretical calculations of braid strength which use the “Ultimate Tensile Strength” of the wire used to make the braid. As the temperature increases, the “Ultimate Tensile Strength” of the wire decreases and this reduces the pressure the braid can handle. For this reason, it is the alloy of the braid, not the hose, which should be used to choose the temperature adjustment factor in the following table.

Additional information and an example of how to calculate the maximum working pressure at elevated temperatures can be found in our Corrugated Metal Hose and Braid Specification Guide.

As temperatures increase above 70°F, the ultimate tensile strength of stainless steel decreases. As temperatures decrease, the ultimate tensile strength actually increases. The information below describes the properties of austenitic stainless steels (All of the stainless steels in the 300 series, like 304, 321 and 316L are austenitic stainless steels) in very low temperatures.

Generally speaking, the austinetic stainless steel, as used in corrugated metal hose, is suitable for applications to the lowest temperatures encountered in cryogenic applications. “Austenitic stainless steels have been used extensively for subzero applications to -269°C (-452°F). The tensile strengths of chromium-nickel austinetic stainless steel increases markedly with decreasing temperature; yield strengths also increase but to a lesser degree. Correspondingly, there is some reduction in ductility as measured by elongation and reduction in area, but ductility values remain high down to the lowest temperature for which data is available. These steels also retain good toughness at -260°C (-452°F).” (1) The fatigue strengths of 300 series stainless steels actually increases as the operating temperature decreases. There is a corresponding decrease in the fatigue, crack growth rates.

In a corrugated metal hose application as the temperature decreases, the following occurs:

• The strength of the hose increases.
• The resistance of the hose to initiation of fatigue cracks increases.
• Fatigue cracks, which are present, or originate, progress through the hose wall at slower rate than at room temperature.

In designing a metal hose assembly for cryogenic service, care must be taken to insure that the fittings and any accessory (guard/liner) materials are suitable for the intended operating temperatures.

Ref: “Stainless Steels” Dec. 1994 American Society for Materials

These two words describe the movement of the hose assembly after it is initially installed. A hose which is installed to take-up a misalignment between a piece of equipment and the piping system outlet, for example, may be bent upon installation but after that it will not move again. It will remain STATIC. Bending the hose to this small radius may result in a permanent “set”, but if the hose will remain in this position, this is not a problem. On the other hand, some hoses are used to accommodate movements which take place repeatedly throughout the life of the hose. These hoses are bent as they are installed in one configuration, but later the position of one end of the hose may move so that the shape of the hose changes to another configuration. This hose is DYNAMIC after it is installed. As the hose shifts from one position to the other, areas along the “curving” sections experience relative movements of the corrugations “compressing” on the inner sides of these arcs and “extending” on the outer sides of the arcs. The smaller the bend radius, the more deflection and stress at each corrugation. For this reason, a larger bend radius is recommended when repeated bending stress is anticipated.