What actually is steel?
The word ‘steel’ refers to all metal alloys whose main component is iron and which can be shaped by forging or rolling. Steel is an alloy (a mixture at atomic level) of the element iron mixed with other metallic or non-metallic elements. The most important alloying element for steel is carbon. Desired characteristics are achieved, and undesired characteristics reduced, by adjusting the amount and types of alloying elements.
According to the classical definition, steel is an iron-carbon alloy that contains less than 2.06% carbon (by mass). Alloys with a higher carbon content are categorised as cast iron; with these, the carbon is in the form of graphite.
What is stainless-steel?
Steel that does not rust is known as stainless-steel, or occasionally as inox steel or simply inox. These forms of steel generally contain at least 12% chromium and are resistant to damage from oxidising agents. Higher chromium content, and additional alloying elements like nickel, molybdenum, titanium or niobium, further improve corrosion resistance. Types of stainless-steel can be divided into two groups, according to their crystalline structure: (i) the ferritic and martensitic steels (magnetic), and (ii) the austenitic steels (non-magnetic). In addition to the main raw material of iron ore, a wide range of other alloying elements may be included. The most important of these are chromium (15.5% to 19%), nickel (8.5% to 13.5%), and molybdenum (2% to 2.5%). In addition, tiny amounts of manganese and titanium may also be added.
Properties of steel:
Typical properties of steel are good workability, durability, high tensile strength and yield point, good heat conductivity and, in the case of stainless-steel, high resistance to corrosion.
Steel, in its various forms, is the most commonly used metal. The most important alloying element in steel is carbon. Its importance in steel lies in its effect on the metal’s properties. In general, steel with a higher carbon content is harder but also more brittle. By alloying with both carbon and other elements, in conjunction with heat and thermo-mechanical treatments (the latter refers to simultaneous heat treatment and physical manipulation), steel’s properties can be modified to suit a wide range of uses. For example, steel can be produced in a very weak and therefore easily shaped form, like for example the steel used for tin cans. Alternatively, it can be produced in a form that is very hard, and therefore brittle, like martensitic steel for knives (cutlery steel). Modern developments aim to produce steel that is simultaneously hard, light and ductile (workable).
Metals (from the Greek word metallon, meaning mine or quarry) are divided into ferric (iron and steel) and non-ferric (e.g. copper, zinc, bronze, or brass). Steel can be further divided into three categories, according to alloying elements and carbon content:
1.) carbon steel – carbon content of less than 0.2%, and no other alloying elements
2.) low-alloy steel – carbon content of more than 0.2% and further alloying elements below 5%
3.) high-alloy steel – individual alloying elements at over 5%.
For specific physical characteristics like high tensile strength, high-alloy steels are always required. Therefore, for our framebuilding only high-alloy steels are used, which best meet the specific requirements of dynamic cycling.
Why we use steel for framebuilding:
Rennstahl bikes are made solely from high-alloy multi-conified tubes, joined by TIG welding, and manufactured in Garching near Munich. This production process has multiple advantages: we can set the geometry according to our own specifications –something that lugged construction does not permit, given that the lugs prescribe the geometry. Also, in contrast with brazed steel frames, this construction method leads to a more robust and torsion-resistant end product. Nevertheless, our steel frames are sufficiently flexible to be able to guarantee the comfort that is typical of steel. In addition, our steel frames use extremely high strength tube sets, which makes reduced tube diameters possible. And our stainless-steel frames stand out for their elegance and simplicity: design reduced to the essential.
There is also the subject of sustainability, which is one of our company’s core values – no superficial ‘greenwash’ here. How we work as a company includes maintaining an awareness of the life-cycles of the materials we use, including how they effect the environment. We keep a careful eye on the ecological balance-sheet of our bikes throughout their manufacturing process, from the energy-efficient extraction of raw materials used to the extremely long life of the end product and its recycleability. The extraction, processing and production of aluminium and carbon fibre often creates highly toxic waste, and accidents can lead to major environmental disasters. Furthermore, the extraction of aluminium often takes place in countries with very dubious working conditions. This is not the case with steel. The mining of iron ore is environmentally and socially unproblematic. And if we include steel’s long service life in the equation, it’s clear that steel protects our scarce resources in the most sustainable way: its production process consumes significantly less energy and scarcely leaves any environmentally-damaging waste.
Why we weld our frames instead of brazing them:
In welding, both parts to be joined are liquefied at the join point and combine to form an inseparable connection. Silver brazing on the other hand involves a binding material being dispersed at the contact points of the parts being joined, which remain unchanged because the working temperature stays well below their melting point. Consequently, the bond is much more easily broken. A welded join can resist much greater forces. However, welding is not an easy process. Weld just a little too hot or too slowly, and you’ve got a hole in the tube. Therefore, frame-builders still building with lugs or doing lugless brazing are usually of the opinion that in a welded frame, material really needs to be added in the area of the weld, because during the melting of the metal its internal structure is changed, which then on cooling has different properties in terms of strength, elasticity, and surface-hardness. Obviously, frame breaks don’t occur in the welded joint itself, as long as it has been done properly, but instead happen next to the joint due to embrittling and fatigue. Not with Rennstahl. Our welds are exceptionally clean and even; weaknesses in the treated area can be ruled out. In addition, we only use steels from the group called the “air-hardening” alloys. With these, the welding process and the subsequent cooling in the air together actually cause the area around the weld to become harder than it was to begin with. For this reason, our frames are exclusively welded.
With TIG welding, the metal is melted using a spark created by a tungsten electrode. Filler metal is added to the “weld pool”, and the two tubes are joined together. At Rennstahl, the process is shielded by an inert gas (argon) in order prevent oxygen bonding with the welded join. In addition, Rennstahl is one of very few manufacturers who also use shielding gas for the inside of the frame, similar to the way titanium frames are built, although this procedure can in theory be skipped to reduce costs. But this extra step adds even more strength to the weld area.
Tube sets used for our frames:
- CroMo 4130 alloy
Our 853 frame set is the flagship amongst steel bike frames. It was developed for tough touring and everyday use, and as it is exceptionally good at absorbing shock from uneven roads it gives a very comfortable ride. As long as the tube dimensions are correctly specified, it is extremely durable. In combination with a good steel fork, this alloy produces the basis of a genuine “carefree bike”.
Our tube set is made from seamless steel tubing. This alloy was originally developed for side-impact protection in the car industry, and belongs to the “air-hardening” category – the special category of steels which after welding and the subsequent cooling process in the air are stronger in the weld area than before. The reason for this is a process that occurs during welding, which changes the grid alignment of the atoms: after the heat treatment the atoms are joined more closely together, which makes the material stronger. Since the critical area for frame breaks is the immediate vicinity of the weld, we knew that this was the tube set to use. It has been optimised and adjusted by Reynolds to suit our requirements: the very best materials, for bikes that are built to last.
Rennstahl 853 frames have another special feature: before painting, they are given a comprehensive priming treatment known as cathodic electro deposition. This involves the frame being given a negative charge within an electric circuit, and the coating to be added is given a positive charge. This renders the complete frame, inside and out, extremely resistant to rust and impact damage. This very demanding process is similar to that used in the production of premium products in large-scale industry. Naturally this comes at a price; just the coating material alone costs between four and six times as much as a standard CroMo coating. But as we all know, details make the difference!
- Stainless Steel
This alloy’s extreme strength makes it possible to produce tubes with walls thinner than ever before: walls as thin as 0.38mm! Its special chemical composition produces a phenomenal durability, simultaneously reducing susceptibility to embrittlement through overheating in the weld area – achieved by maintaining the alignment of the fine-grained molecular structure.
The 931 tube set is only fractionally heavier than 6061 aluminium, while still retaining the typical strength, durability and ride characteristics of steel. In addition, 931 frames are completely corrosion-resistant, which means that painting is not required. Instead we subject our 931 frames to a comprehensive wire-brushing and polishing process, before adding the lettering using a sand-blasting process involving a special mixture of quartz sand and glass beads. The result: no paint, no decals, just pure minimalism! A bike that in its reduction to the essentials is without equal.
Due to this tube set’s focus on extreme weight advantage, it is particularly good for frames that are built for racing. The capabilities of our 931 frames match those of carbon fibre frames, but without their infamous fragility.
Working with state-of-the-art materials does of course affect the price. Using insights obtained from our experience with titanium frame-building, we have also had the 931 tube set optimised. As a result, just the raw tube set costs ten times as much as equivalents made from standard CroMo. So, as you see, not all steels are the same!