What grade steel is HSS?

Type of metal profile

"Structural tubing" redirects here. For other uses, see Tubing

A hot-rolled HSS-box section cut open with a bandsaw, demonstrating residual stress

A hollow structural section (HSS) is a type of metal profile with a hollow cross section. The term is used predominantly in the United States, or other countries which follow US construction or engineering terminology.

HSS members can be circular, square, or rectangular sections, although other shapes such as elliptical are also available. HSS is only composed of structural steel per code.

HSS is sometimes mistakenly referenced as hollow structural steel. Rectangular and square HSS are also commonly called tube steel or box section. Circular HSS are sometimes mistakenly called steel pipe, although true steel pipe is actually dimensioned and classed differently from HSS. (HSS dimensions are based on exterior dimensions of the profile; pipes are also manufactured to an exterior tolerance, albeit to a different standard.) The corners of HSS are heavily rounded, having a radius which is approximately twice the wall thickness. The wall thickness is uniform around the section.

In the UK, or other countries which follow British construction or engineering terminology, the term HSS is not used. Rather, the three basic shapes are referenced as CHS, SHS, and RHS, being circular, square, and rectangular hollow sections. Typically, these designations will also relate to metric sizes, thus the dimensions and tolerances differ slightly from HSS.

Use in structures

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HSS, especially rectangular sections, are commonly used in welded steel frames where members experience loading in multiple directions. Square and circular HSS have very efficient shapes for this multiple-axis loading as they have uniform geometry along two or more cross-sectional axes, and thus uniform strength characteristics. This makes them good choices for columns. They also have excellent resistance to torsion.

HSS can also be used as beams, although wide flange or I-beam shapes are in many cases a more efficient structural shape for this application. However, the HSS has superior resistance to lateral torsional buckling.

The flat square surfaces of rectangular HSS can ease construction, and they are sometimes preferred for architectural aesthetics in exposed structures, although elliptical HSS are becoming more popular in exposed structures for the same aesthetic reasons.

In the recent past, HSS was commonly available in mild steel, such as A500 grade B. Today, HSS is commonly available in mild steel, A500 grade C. Other steel grades available for HSS are A847 (weathering steel), A1065 (large sections up to 50 inch sq made with SAW process), and recently approved A1085 (higher strength, tighter tolerances than A500).

Manufacture

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Square HSS is made the same way as pipe. During the manufacturing process flat steel plate is gradually changed in shape to become round where the edges are presented ready to weld. The edges are then welded together to form the mother tube. During the manufacturing process the mother tube goes through a series of shaping stands which form the round HSS (mother tube) into the final square or rectangular shape. Most American manufacturers adhere to the ASTM A500 or newly adopted ASTM A1085 standards, while Canadian manufacturers follow both ASTM A500 and CSA G40.21. European hollow sections are generally in accordance with the EN 10210 standard.

Filled HSS

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HSS is often filled with concrete to improve fire rating, as well as robustness. When this is done, the product is referred to as a Lally column after its inventor John Lally of Waltham, Massachusetts. (The pronunciation is often corrupted to lolly column.) For example, barriers around parking areas (bollards) made of HSS are often filled, to at least bumper height, with concrete. This is an inexpensive (when replacement costs are factored in) way of adding compressive strength to the bollard, which can help prevent unsightly local denting, though it does not generally significantly increase the overall structural properties of the bollard.

See also

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Subset of tool steels

High-speed steel (HSS or HS) is a subset of tool steels, commonly used as cutting tool material.

It is often used in power-saw blades and drill bits. In addition, it is often used in bowl gouges and skew for woodturning.[1] It is superior to the older high-carbon steel tools used extensively through the 1940s in that it can withstand higher temperatures without losing its temper (hardness). This property allows HSS to cut faster than high carbon steel, hence the name high-speed steel. At room temperature, in their generally recommended heat treatment, HSS grades generally display high hardness (above Rockwell hardness 60) and abrasion resistance (generally linked to tungsten and vanadium content often used in HSS) compared with common carbon and tool steels. There are several different types of HS steel, such as M42 and M2.[1]

History

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In 1868 English metallurgist Robert Forester Mushet developed Mushet steel, considered the forerunner of modern high-speed steels. It consisted of 2% carbon, 2.5% manganese, and 7% tungsten. The major advantage of this steel was that it hardened when air cooled from a temperature at which most steels had to be quenched for hardening. Over the next 30 years, the most significant change was the replacement of manganese with chromium.[2]

In 1899 and 1900, Frederick Winslow Taylor and Maunsel White (né Maunsel White III; 1856–1912; grandson of Maunsel White; 1783–1863), working with a team of assistants at the Bethlehem Steel Company at Bethlehem, Pennsylvania, US, performed a series of experiments with heat treating existing high-quality tool steels, such as Mushet steel, heating them to much higher temperatures than were typically considered desirable in the industry.[3][4] Their experiments were characterised by a scientific empiricism in that many different combinations were made and tested, with no regard for conventional wisdom or alchemic recipes, and detailed records kept of each batch. The result was a heat treatment process that transformed existing alloys into a new kind of steel that could retain its hardness at higher temperatures, allowing much higher speeds and rate of cutting when machining.

The Taylor-White process[5] was patented and created a revolution in machining industries. Heavier machine tools with higher rigidity were needed to use the new steel to its full advantage, prompting redesigns and replacement of installed plant machinery. The patent was contested and eventually nullified.[6]

The first alloy that was formally classified as high-speed steel is known by the AISI designation T1, which was introduced in 1910.[7] It was patented by Crucible Steel Co. at the beginning of the 20th century.[2]

Although molybdenum-rich high-speed steels such as AISI M1 had seen some use since the 1930s, it was the material shortages and high costs caused by WWII that spurred development of less expensive alloys substituting molybdenum for tungsten. The advances in molybdenum-based high speed steel during this period put them on par with, and in certain cases better, than tungsten-based high speed steels. This started with the use of M2 steel instead of T1 steel.[2][8]

Types

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High speed steels are alloys that gain their properties from a variety of alloying metals added to carbon steel, typically including tungsten and molybdenum, or a combination of the two, often with other alloys as well.[9] They belong to the Fe–C–X multi-component alloy system where X represents chromium, tungsten, molybdenum, vanadium, or cobalt. Generally, the X component is present in excess of 7%, along with more than 0.60% carbon.

In the unified numbering system (UNS), tungsten-type grades (e.g. T1, T15) are assigned numbers in the T120xx series, while molybdenum (e.g. M2, M48) and intermediate types are T113xx. ASTM standards recognize 7 tungsten types and 17 molybdenum types.[10]

The addition of about 10% of tungsten and molybdenum in total maximises efficiently the hardness and toughness of high speed steels and maintains those properties at the high temperatures generated when cutting metals.

A sample of alloying compositions of common high speed steel grades (by %wt)[11][12] (impurity limits are not included) Grade C Cr Mo W V Co Mn Si T1 0.65–0.80 4.00 - 18 1 - 0.1–0.4 0.2–0.4 M1 0.80 4 8 1.5 1.0 - - - M2 0.85 4 5 6.0 2.0 - - - M7 1.00 4 8.75 1.75 2.0 - - - M35 0.92 4.3 5 6.4 1.8 5 - 0.35 M42 1.10 3.75 9.5 1.5 1.15 8.0 - - M50 0.85 4 4.25 .10 1.0 - - -

Molybdenum High Speed Steels (HSS)

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Combining molybdenum, tungsten and chromium steel creates several alloys commonly called "HSS", measuring 63–65 Rockwell "C" hardness.

M1

M1 lacks some of the red-hardness properties of M2, but is less susceptible to shock and will flex more.

M2

M2 is the "standard" and most widely used industrial HSS. It has small and evenly distributed carbides giving high wear resistance, though its decarburization sensitivity is a little bit high. After heat treatment, its hardness is the same as T1, but its bending strength can reach 4700 MPa, and its toughness and thermo-plasticity are higher than T1 by 50%. It is usually used to manufacture a variety of tools, such as drill bits, taps and reamers. 1.3343 is the equivalent numeric designation for M2 material identified in ISO 4957.

M7

M7 is used for making heavier construction drills where flexibility and extended drill life are equally important.

M50

M50 does not have the red-hardness of other grades of tungsten HSS, but is very good for drills where breakage is a problem due to flexing the drill. Generally favored for hardware stores and contractor use. It is also used in high-temperature ball bearings.

Cobalt High Speed Steels (HSS)

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The addition of cobalt increases heat resistance, and can give a Rockwell hardness up to 70 Min.[13]

M35

M35 is similar to M2, but with 5% cobalt added. M35 is also known as Cobalt Steel, HSSE or HSS-E. It will cut faster and last longer than M2.[14]

M42

M42 is a molybdenum-series high-speed steel alloy with an additional 8% cobalt.[13] It is widely used in metal manufacturing industries because of its superior red-hardness as compared to more conventional high-speed steels, allowing for shorter cycle times in production environments due to higher cutting speeds or from the increase in time between tool changes.[14]

Forming

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HSS drill bits formed by rolling are denoted HSS-R. Grinding is used to create HSS-G, cobalt and carbide drill bits.[15]

Applications

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The main use of high-speed steels continues to be in the manufacture of various cutting tools: drills, taps, milling cutters, tool bits, hobbing (gear) cutters, saw blades, planer and jointer blades, router bits, etc., although usage for punches and dies is increasing.

High speed steels also found a market in fine hand tools where their relatively good toughness at high hardness, coupled with high abrasion resistance, made them suitable for low speed applications requiring a durable keen (sharp) edge, such as files, chisels, hand plane blades, and damascus kitchen knives and pocket knives.[citation needed]

High speed steel tools are the most popular for use in woodturning, as the speed of movement of the work past the edge is relatively high for handheld tools, and HSS holds its edge far longer than high carbon steel tools can.[citation needed]

See also

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References

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