Each test protocol (Brinell, Rockwell, Vickers) has procedures specific to the object under test. The Rockwell t-test is useful for testing thin-walled pipes by cutting the pipe lengthwise and checking the pipe wall by the inside diameter rather than the outside diameter.
Ordering pipes is a bit like going to a car dealership and ordering a car or truck. There are now a plethora of options available that allow buyers to customize the car in a variety of ways – interior and exterior colors, trim packages, exterior styling options, powertrain choices, and an audio system that’s nearly as good as a home entertainment system. With all these options, you probably won’t be satisfied with a standard no-frills car.
This applies to steel pipes. It has thousands of options or specifications. In addition to dimensions, the specification mentions chemical properties and several mechanical properties such as minimum yield strength (MYS), ultimate tensile strength (UTS), and minimum elongation to failure. However, many in the industry—engineers, purchasing agents, and manufacturers—use the industry’s shorthand and call for “simple” welded pipes and list only one characteristic: hardness.
Try to order a car according to one characteristic (“I need a car with an automatic transmission”), and with the seller you will not go far. He has to fill out a form with a lot of options. This is the case with steel pipes: in order to get a pipe suitable for an application, a pipe manufacturer needs much more information than hardness.
How did hardness become an accepted substitute for other mechanical properties? It probably started with pipe manufacturers. Because hardness testing is quick, easy, and requires relatively inexpensive equipment, pipe sellers often use hardness testing to compare two types of pipe. All they need to perform a hardness test is a smooth piece of pipe and a test rig.
Pipe hardness is closely related to UTS and a rule of thumb (percentage or percentage range) is useful for estimating MYS, so it is easy to see how hardness testing can be a suitable proxy for other properties.
In addition, other tests are relatively difficult. While hardness testing takes only about a minute on a single machine, MYS, UTS and elongation tests require sample preparation and a significant investment in large laboratory equipment. In comparison, a pipe mill operator completes a hardness test in seconds, while a specialist metallurgist performs a tensile test in a few hours. Performing a hardness test is not difficult.
This does not mean that engineering pipe manufacturers do not use hardness tests. It’s safe to say that the majority do this, but since they evaluate instrument repeatability and reproducibility across all test equipment, they are well aware of the limitations of the test. Most of them use it to evaluate the hardness of the tube as part of the manufacturing process, but do not use it to quantify the properties of the tube. It’s just a pass/fail test.
Why do I need to know MYS, UTS and minimum elongation? They indicate the performance of the tube assembly.
MYS is the minimum force that causes permanent deformation of the material. If you try to slightly bend a straight piece of wire (like a hanger) and release the pressure, one of two things will happen: it will return to its original state (straight) or stay bent. If it’s still straight, then you haven’t gotten over MYS yet. If it’s still bent, you missed.
Now grab both ends of the wire with pliers. If you can break a wire in half, you’ve made it past UTS. You pull it hard and you have two pieces of wire to show your superhuman efforts. If the original length of the wire was 5 inches, and the two lengths after failure add up to 6 inches, the wire will stretch 1 inch, or 20%. Actual tensile tests are measured within 2 inches of the break point, but no matter what – the line tension concept illustrates UTS.
Steel micrograph specimens must be cut, polished, and etched with a weakly acidic solution (usually nitric acid and alcohol) to make the grains visible. 100x magnification is commonly used to inspect steel grains and determine their size.
Hardness is a test of how a material reacts to impact. Imagine that a short length of tubing is placed in a vise with serrated jaws and shaken to close the vise. In addition to aligning the pipe, vise jaws leave an imprint on the surface of the pipe.
This is how the hardness test works, but it’s not as rough. The test has a controlled impact size and a controlled pressure. These forces deform the surface, forming indentations or indentations. The size or depth of the dent determines the hardness of the metal.
When evaluating steel, Brinell, Vickers and Rockwell hardness tests are commonly used. Each one has its own scale, and some of them have multiple test methods such as Rockwell A, B, C, etc. For steel pipes, the ASTM A513 specification refers to the Rockwell B test (abbreviated as HRB or RB). The Rockwell Test B measures the difference in penetration force of a 1⁄16 inch diameter steel ball into steel between a light preload and a basic load of 100 kgf. A typical result for standard mild steel is HRB 60.
Materials scientists know that hardness has a linear relationship with UTS. Therefore, the given hardness predicts UTS. Similarly, the pipe manufacturer knows that MYS and UTS are related. For welded pipes, MYS is typically 70% to 85% UTS. The exact amount depends on the tube manufacturing process. The hardness of HRB 60 corresponds to UTS 60,000 pounds per square inch (PSI) and about 80% MYS, which is 48,000 PSI.
The most common pipe specification for general production is maximum hardness. In addition to size, engineers are also interested in specifying resistance welded (ERW) pipes within a good operating range, which can result in part drawings with a possible maximum hardness of HRB 60. This decision alone results in a number of mechanical end properties, including hardness itself.
First, the hardness of HRB 60 doesn’t tell us much. The HRB 60 reading is a dimensionless number. Materials rated at HRB 59 are softer than those tested at 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), speed (measured in distance versus time), or UTS (measured in pounds per square inch). Reading HRB 60 doesn’t tell us anything specific. It is a material property, not a physical property. Secondly, the determination of hardness by itself is not well suited to ensure repeatability or reproducibility. Evaluation of two sites on a sample, even if the test sites are close together, often results in very different hardness readings. The nature of tests exacerbates this problem. After one position measurement, a second measurement cannot be taken to check the result. Test repeatability is not possible.
This does not mean that hardness measurement is inconvenient. Actually, this is a good guide to UTS stuff, and it’s a quick and easy test. However, anyone involved in the definition, procurement, and manufacture of tubes should be aware of their limitations as a test parameter.
Because “regular” pipe is not clearly defined, pipe manufacturers typically narrow it down to the two most commonly used types of steel and pipe as defined in ASTM A513:1008 and 1010 when appropriate. Even after excluding all other types of pipes, the possibilities for the mechanical properties of these two types of pipes remain open. In fact, these types of pipes have the widest range of mechanical properties of all pipe types.
For example, a tube is considered soft if MYS is low and elongation is high, which means that it performs better in terms of stretch, deformation, and permanent deformation than a tube described as rigid, which has a relatively high MYS and relatively low elongation. . This is similar to the difference between soft wire and hard wire such as clothes hangers and drills.
Elongation itself is another factor that has a significant impact on critical pipe applications. High elongation pipes can withstand stretching; low elongation materials are more brittle and therefore more prone to catastrophic fatigue failure. However, elongation is not directly related to UTS, which is the only mechanical property directly related to hardness.
Why do pipes vary so much in their mechanical properties? First, the chemical composition is different. Steel is a solid solution of iron and carbon, as well as other important alloys. For simplicity, we will deal only with the percentage of carbon. The carbon atoms replace some of the iron atoms, creating the crystalline structure of the steel. ASTM 1008 is a comprehensive primary grade with carbon content from 0% to 0.10%. Zero is a special number that provides unique properties at an ultra-low carbon content in steel. ASTM 1010 defines carbon content from 0.08% to 0.13%. These differences don’t seem huge, but they are enough to make a big difference elsewhere.
Secondly, steel pipes can be manufactured or manufactured and subsequently processed in seven different manufacturing processes. ASTM A513 regarding the production of ERW pipes lists seven types:
If the chemical composition of steel and the stages of pipe manufacturing do not affect the hardness of steel, then what? The answer to this question means careful study of the details. This question leads to two other questions: what details and how close?
Detailed information about the grains that make up steel is the first answer. When steel is produced in a primary mill, it does not cool into a huge mass with one property. As steel cools, its molecules form repeating patterns (crystals), similar to how snowflakes form. After the formation of crystals, they are combined into groups called grains. As the grains cool, they grow, forming the entire sheet or plate. Grain growth stops when the last molecule of steel is absorbed by the grain. This all happens on a microscopic level, with a medium-sized steel grain being about 64 microns or 0.0025 inches across. While each grain is similar to the next, they are not the same. They differ slightly from each other in size, orientation, and carbon content. The interfaces between grains are called grain boundaries. When steel fails, for example due to fatigue cracks, it tends to fail at grain boundaries.
How close do you have to look to see distinct particles? A magnification of 100 times or 100 times the visual acuity of the human eye is sufficient. However, simply looking at raw steel to the 100th power doesn’t do much. Samples are prepared by polishing the sample and etching the surface with an acid, usually nitric acid and alcohol, which is called nitric acid etching.
It is the grains and their internal lattice that determine the impact strength, MYS, UTS, and the elongation that the steel can withstand before failure.
Steelmaking steps such as hot and cold strip rolling transfer stress to the grain structure; if they constantly change shape, this means that the stress has deformed the grains. Other processing steps such as winding the steel into coils, unwinding and passing through a tube mill (to form the tube and size) deform the steel grains. The cold drawing of the pipe on the mandrel also stresses the material, as do the manufacturing steps such as end forming and bending. Changes in the grain structure are called dislocations.
The above steps deplete the steel’s ductility, its ability to withstand tensile (tearing) stress. Steel becomes brittle, which means that it is more likely to break if you continue to work with the steel. Elongation is one component of plasticity (compressibility is another). It is important to understand here that failure most often occurs in tension, and not in compression. Steel is quite resistant to tensile stresses due to its relatively high elongation. However, steel easily deforms under compressive stress—it is malleable—which is an advantage.
Compare this to concrete, which has very high compressive strength but low ductility. These properties are opposite to steel. This is why concrete used for roads, buildings and sidewalks is often reinforced. The result is a product that has the strengths of both materials: steel is strong in tension and concrete is strong in compression.
During hardening, the ductility of steel decreases, and its hardness increases. In other words, it hardens. Depending on the situation, this can be an advantage, but it can also be a disadvantage, as hardness equates to brittleness. That is, the harder the steel, the less elastic it is and therefore the more likely it is to fail.
In other words, each step of the process requires some pipe ductility. As the part is processed, it becomes heavier, and if it is too heavy, then in principle it is useless. Hardness is brittleness, and brittle tubes are prone to failure during use.
Does the manufacturer have options in this case? In short, yes. This option is annealing, and while not exactly magical, it is about as magical as can be.
In simple terms, annealing removes all the effects of physical impact on metals. In the process, the metal is heated to a stress relief or recrystallization temperature, which results in the removal of dislocations. Thus, the process partially or completely restores ductility, depending on the specific temperature and time used in the annealing process.
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 almost magical thing. Quenching at the right temperature with the right hardening agent at the right time quickly stops the process and restores the properties of the steel.
Should we abandon hardness specifications? no. The properties of hardness are valuable, first of all, as a guideline in determining the characteristics of steel pipes. Hardness is a useful measurement and one of several properties that should be specified when ordering tubular material and checked upon receipt (documented for each shipment). When a hardness test is used as a test standard, it must have appropriate scale values and control limits.
However, this is not a true test of passing (acceptance or rejection) of the material. In addition to hardness, manufacturers should check shipments from time to time to determine other relevant properties such as MYS, UTS, or minimum elongation, depending on the pipe application.
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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Post time: Jan-27-2023