Various test protocols (Brinell, Rockwell, Vickers) have procedures specific to the project under test.The Rockwell T test is suitable for inspecting light wall tubes by cutting the tube lengthwise and testing the wall from the inner diameter rather than the outer diameter.
Ordering a tubing is a bit like going to a car dealership and ordering a car or truck.Today, the many options available allow buyers to customize the vehicle in a variety of ways — interior and exterior colors, interior trim packages, exterior styling options, powertrain choices, and an audio system that nearly rivals a home entertainment system.Given all these options, you may not be satisfied with a standard, no-frills vehicle.
Steel pipes are just that.It has thousands of options or specifications.In addition to dimensions, the specification lists chemical and several mechanical properties such as minimum yield strength (MYS), ultimate tensile strength (UTS), and minimum elongation before failure.However, many in the industry—engineers, purchasing agents, and manufacturers—use accepted industry shorthands that require the use of “normal” welded pipe and specify only one characteristic: hardness.
Try ordering a car by a single characteristic (“I need a car with an automatic transmission”) and you won’t get too far with a salesman.He has to fill out an order form with many options.Pipe is just that – in order to get the right pipe for the application, the pipe manufacturer needs more information than just hardness.
How does hardness become a recognized substitute for other mechanical properties?It probably started with a pipe producer.Because hardness testing is quick, easy, and requires relatively inexpensive equipment, tube salespeople often use hardness testing to compare two tubes.To perform a hardness test, all they need is a smooth length of pipe and a test stand.
Tube hardness correlates well with UTS, and as a rule of thumb, percentages or percentage ranges are helpful in estimating MYS, so it’s easy to see how hardness testing can be a suitable proxy for other properties.
Also, other tests are relatively complex.While hardness testing takes only a minute or so on a single machine, MYS, UTS and elongation testing requires sample preparation and significant investment in large laboratory equipment.As a comparison, it takes seconds for a tube mill operator to perform a hardness test and hours for a professional metallurgical technician to perform a tensile test.It is not difficult to perform a hardness check.
This is not to say that engineered pipe manufacturers do not use hardness testing.It’s safe to say that most people do, but because they do gage repeatability and reproducibility assessments on all their test equipment, they are well aware of the limitations of the test.Most use assessing tube hardness as part of the production process, but they do not use it to quantify tube properties.This is just a pass/fail test.
Why do you need to know about MYS, UTS and minimum elongation?They indicate how the tube will behave in assembly.
MYS is the minimum force that causes permanent deformation of the material.If you try to bend a straight wire (like a coat hanger) slightly and release the pressure, one of two things will happen: it will spring back to its original state (straight) or it will remain bent.If it’s still straight, you haven’t gotten past MYS.If it’s still bent, you’ve overshot it.
Now, use pliers to clamp both ends of the wire.If you can tear the wire into two pieces, you’re over its UTS.You put a lot of tension on it and you have two wires to show your superhuman effort.If the original length of the wire is 5 inches, and the two lengths after failure add up to 6 inches, the wire is stretched by 1 inch, or 20%.The actual elongation test is measured within 2 inches of the point of failure, but whatever – the pull wire concept illustrates the UTS.
Steel photomicrograph samples need to be cut, polished, and etched using a mildly acidic solution (usually nitric acid and alcohol (nitroethanol)) to make the grains visible.100x magnification is commonly used to inspect steel grains and determine grain size.
Hardness is a test of how a material responds to impact.Imagine putting a short piece of pipe into a vise with serrated jaws and turning the vise to close.In addition to flattening the tube, the jaws of the vise also leave indentations on the surface of the tube.
That’s how the hardness test works, but it’s not that rough.This test has a controlled impact size and controlled pressure.These forces deform the surface, creating an indentation or indentation.The size or depth of the indentation determines the hardness of the metal.
For evaluating steel, common hardness tests are Brinell, Vickers, and Rockwell.Each has its own scale, and some have multiple test methods, such as Rockwell A, B, and C.For steel pipes, ASTM Specification A513 references the Rockwell B test (abbreviated as HRB or RB).The Rockwell B test measures the difference in penetration of steel by a 1⁄16-inch diameter steel ball between a small preload and a primary load of 100 kgf.A typical result for standard mild steel is HRB 60.
Materials scientists know that hardness is linearly related to UTS.Therefore, a given hardness can predict UTS.Likewise, tube manufacturers know that MYS and UTS are related.For welded pipe, MYS is typically 70% to 85% of UTS.The exact amount depends on the process of making the tube.The hardness of HRB 60 correlates to a UTS of 60,000 pounds per square inch (PSI) and a MYS of 80%, or 48,000 PSI.
The most common pipe specification in general manufacturing is maximum hardness.In addition to size, the engineer was concerned with specifying a welded electric resistance welded (ERW) pipe within a good working range, which could result in a maximum hardness of possibly HRB 60 finding its way on the component drawing.This decision alone leads to a range of final mechanical properties, including hardness itself.
First, the hardness of HRB 60 doesn’t tell us much.The reading HRB 60 is a dimensionless number.The material evaluated with HRB 59 is softer than the material tested with HRB 60, and HRB 61 is harder than HRB 60, but by how much?It cannot be quantified like volume (measured in decibels), torque (measured in pound-feet), velocity (measured in distance relative to time), or UTS (measured in pounds per square inch).Reading HRB 60 doesn’t tell us anything specific.This is a property of the material, but not a physical property.Second, hardness testing is not suitable for repeatability or reproducibility.Evaluating two locations on a test specimen, even if the test locations are close to each other, often results in a large variation in hardness readings.Compounding this issue is the nature of the test.After a position has been measured, it cannot be measured a second time to verify the results.Test repeatability is not possible.
This does not mean that hardness testing is inconvenient.In fact, it provides a good guide for a material’s UTS, and it’s a quick and easy test to perform.However, everyone involved in specifying, purchasing and manufacturing tubes should be aware of its limitations as a test parameter.
Because “normal” pipe is not well defined, when needed, pipe manufacturers often narrow it down to the two most commonly used steel pipe and pipe types defined in ASTM A513: 1008 and 1010.Even after eliminating all other tube types, the possibilities in terms of mechanical properties of these two tube types are wide open.In fact, these tube types have the widest range of mechanical properties of any type.
For example, a tube is described as soft if MYS is low and elongation is high, which means that it performs better in tensile, deflection and set than a tube described as hard, which has a relatively high MYS and relatively low elongation.This is similar to the difference between soft and hard wire, such as coat hangers and drills.
Elongation itself is another factor that has a significant impact on critical pipe applications.Tubes with high elongation can withstand tensile forces; materials with low elongation are more brittle and therefore more prone to catastrophic fatigue-type failures.However, elongation is not directly related to UTS, which is the only mechanical property directly related to hardness.
Why do the mechanical properties of the tubes vary so much?First, the chemical composition is different.Steel is a solid solution of iron and carbon and other important alloys.For simplicity, we will only deal with carbon percentages here.Carbon atoms replace some of the iron atoms, forming the crystal structure of steel.ASTM 1008 is an all-encompassing primary grade with a carbon content of 0% to 0.10%.Zero is a very special number that produces unique properties when the carbon content in steel is ultra-low.ASTM 1010 specifies a carbon content between 0.08% and 0.13%.These differences don’t seem huge, but they’re big enough to make a big difference elsewhere.
Second, the steel pipe can be fabricated or fabricated and subsequently processed in seven different manufacturing processes.ASTM A513 related to ERW pipe production lists seven types:
If the chemical composition of the steel and the tube manufacturing steps have no effect on the hardness of the steel, what is?Answering this question means poring over the details.This question begs two more questions: What details, and how close?
Details about the grains that make up the steel are the first answer.When steel is made at a primary steel mill, it does not cool into a huge block with a single feature.As the steel cools, the steel’s molecules organize in repeating patterns (crystals), similar to how snowflakes form.After crystals are formed, they aggregate into groups called grains.As cooling progresses, grains grow and form throughout the sheet or plate.The grains stop growing as the last steel molecules are absorbed by the grains.All of this happens at the microscopic level because the average size steel grain is about 64 µ or 0.0025 inches wide.While each grain is similar to the next, they are not the same.They vary slightly in size, orientation and carbon content.The interface between grains is called grain boundary.When steel fails, for example due to fatigue cracks, it tends to fail along grain boundaries.
How far do you have to look to see discernible grains?100x magnification, or 100x human vision, is enough.However, just looking at untreated steel at 100 times the power doesn’t reveal much.The sample is prepared by polishing the sample and etching the surface with an acid (usually nitric acid and alcohol) called a nitroethanol etchant.
It is the grains and their internal lattice that determine the impact strength, MYS, UTS and elongation a steel can withstand before failure.
Steelmaking steps, such as hot and cold rolling of strip, apply stress into the grain structure; if they permanently change shape, this means that the stress deforms the grain.Other processing steps, such as coiling the steel into coils, uncoiling it, and deforming the steel grains through a tube mill (to form and size the tube).Cold drawing the tube on the mandrel also puts pressure on the material, as does manufacturing steps such as end forming and bending.Changes in grain structure are called dislocations.
The above steps deplete the ductility of the steel, which is its ability to withstand tensile (pull-open) stress.Steel becomes brittle, which means it’s more likely to break if you keep working on it.Elongation is one component of ductility (compressibility is another).It is important to understand that failure most often occurs during tensile stress, not compression.Steel is very resistant to tensile stress because of its relatively high elongation capacity.However, steel deforms easily under compressive stress – it is ductile – which is an advantage.
Concrete has high compressive strength but low ductility compared to concrete.These properties are opposite to those of steel.That’s why concrete used for roads, buildings and sidewalks is often fitted with rebar.The result is a product with the strengths of two materials: under tension, steel is strong, and under pressure, concrete.
During cold working, as the ductility of the steel decreases, its hardness increases.In other words, it will harden.Depending on the situation, this may be a benefit; however, it may be a disadvantage since hardness is equated with brittleness.That is, as steel becomes harder, it becomes less elastic; therefore, it is more likely to fail.
In other words, each process step consumes some of the pipe’s ductility.It gets harder as the part works, and if it’s too hard it’s basically useless.Hardness is brittleness, and a brittle tube is likely to fail when used.
Does the manufacturer have any options in this case?In short, yes.That option is annealing, and while it’s not quite magical, it’s as close to magic as you can get.
In layman’s terms, annealing removes all effects of physical stress on the metal.This process heats the metal to a stress-relief or recrystallization temperature, thereby eliminating dislocations.Depending on the specific temperature and time used in the annealing process, the process thus restores some or all of its ductility.
Annealing and controlled cooling promote grain growth.This is beneficial if the goal is to reduce the brittleness of the material, but uncontrolled grain growth can soften the metal too much, rendering it unusable for its intended use.Stopping the annealing process is another near-magical thing.Quenching at the right temperature with the right quenching agent at the right time brings the process to a quick stop to get the steel’s recovery properties.
Should we drop the hardness specification?no.Hardness characteristics are valuable primarily as a reference point when specifying steel pipes.A useful measure, hardness is one of several characteristics that should be specified when ordering tubular material and checked upon receipt (and should be recorded with each shipment).When hardness inspection is the inspection standard, it should have appropriate scale values ​​and control ranges.
However, it is not a true test for qualifying (accepting or rejecting) material.In addition to hardness, manufacturers should occasionally test shipments to determine other relevant properties, such as MYS, UTS, or minimum elongation, depending on the application of the tube.
Wynn H. Kearns is responsible for regional sales for Indiana Tube Corp., 2100 Lexington Road, Evansville, IN 47720, 812-424-9028, wkearns@indianatube.com, www.indianatube.com.
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