Q: What is a stainless steel?

A: It is a steel alloy that contains at least 10.5% chromium. The chromium and oxygen forms a passive chromium oxide layer that protects the metal from corroding.


Q: What is “acid-proof steel”?

A: Acid-proof steel is a layman term that is used to describe steel that are alloyed with molybdenum and thereby has improved corrosion resistance. The most common acid-proof steel is ASTM 316 and most of the grades in the Bumax portfolio can be considered as acid-proof steel. It is however important to point out that the term “acid-proof steel” does not necessary mean that the material is corrosion resistant in acid, it is more complicated than that. Chemical composition of the solution, concentration and temperature must to be considered. Bufab can help you with selecting the best possible material for each environment.


Q: What is duplex stainless steel?

A: Duplex stainless steel, also known as Ferrite-Austenitic stainless steel, has a mixed microstructure of austenite and ferrite. It is a family that consists of high strength corrosion resistant grades and are characterized into groups based on their alloy content and corrosion resistance.

  • Lean duplex, similar or lower corrosion resistance than ASTM 316
  • Standard Duplex, most common type of duplex stainless steel, with better corrosion resistance than ASTM 316
  • Super Duplex, PRE >40, material for sever corrosive environments such as petrochemical, oil/gas and marine.
  • Hyper Duplex, PRE >48, better corrosion resistance than all super duplex stainless steel grades available on the market


Q: What is PRE?

A: PRE stands for Pitting Resistance Equivalent. It is a very common way of ranking different stainless steel grades in regards of pitting corrosion resistance, mainly in saltwater or chloride bearing environments. It exist several different types of equations, but the most common one is:

PRE = Cr% + 3.3 x Mo% + 16 x N%


Q: Are all BUMAX products from European origin?

A: Yes, all BUMAX are produced in Europe. The pre-material (wire rod, bars and sheets) for BUMAX comes solely from European premium manufacturers.


Q: Why should I use BUMAX 88 instead of an ordinary A4-80?

A: Bumax 88 is a premium product within the A4 standard. Bumax 88 offers many advantages compared to A4 and the main differences between BUMAX 88 and the vast majority of A4 fasteners are:

  • Bumax 88 is a 316LHigh Mo grade, meaning that we guarantee a minimum quantity of 2.5% molybdenum compared to a standard A4 which is 2.0%. That has a positive impact on the PRE value and corrosion resistance.
  • Bumax 88 is sourced solely from premium European stainless steel manufacturers according to rigid specifications. The tight tolerance on each element, controlled process and the best available metallurgical process gives very clean steel with low amount of inclusions and slags. Resulting in a consistent material with very little variation from batch to batch and much better fatigue and corrosion resistance.
  • Bumax 88 has higher yield strength than A4. Bumax 88 offers minimum yield strength of 640 MPa, to be compared with A4 that has minimum 600 MPa.
  • Bumax 88 offers traceability through the whole production process, from melting to finished fastener.
  • Bumax 88 offers excellent surface quality and hard threads. Many customers appreciate the anti-galling properties of Bumax 88 compared to A4


Q: What is the difference between cut and rolled thread?

A: Cut threading is a process that normally takes place in a lathe where steel is cut away from a round bar to form the threads. Roll threading is a process by which the material is extruded with threading dies to form the thread, instead of being removed. There are several benefits with rolled threading and it is used whenever possible on Bumax fasteners. The strain hardening that takes place during roll threading will increase the strength of the fastener and is beneficial for the fatigue resistance. Furthermore, rolled threads are often smoother than cut threads. There are some products where rolled threading cannot be used and cutting is needed, mainly for very large dimensions.


Q: Can Bumax be manufactured according customer specific drawings and requirements?

A: Yes, it is part of our business model to offer solutions to specific customer requirements. We have an engineering department that are able to offer design guidance, calculation support, testing and material selecting support.


Q: What is the maximum service temperature for BUMAX?

A: Recommended maximum service temperature for each grade can be found in the section “Technical information”. All steels are affected by elevated temperatures, which can lead to reduced strength, fatigue resistance and corrosion resistance. Long-time exposure can also cause creep deformation, relaxation and brittleness due to over-aged material. The thermal expansion that occurs at elevated temperatures is also something that must be considered, especially if material with different Constant of Thermal Expansion is combined. It is for the majority of applications safe to go up to recommended maximal service temperature, with limited effect on material properties. It exist cases where Bumax fasteners has successfully been used in temperatures exceeding recommended maximum service temperature, but that must be evaluated from case to case with good understanding on the effect it will have on the fastener properties.


Q: Are BUMAX products non-magnetic?

A: We have grades that are optimized to get very low magnetic permeability, and therefore be considered as non-magnetic. Austenitic stainless steel has generally very low magnetic permeability but is affected by chemical composition, metallurgical process, residual stresses and amount of deformation martensite that is created during cold working. Our Bumax 88 is well suited for applications with high demands on low magnetic permeability, such as electrical industry and nuclear power. Bumax 88 offers lower magnetic permeability than standard A4, with less variation in properties from batch to batch. For more information see section “Grades- Physical properties” or contact Bufab.

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