Introduction to Stainless Steels
It comes as a surprise to many that ‘stainless steel’ is not stainless. Discolouration is not uncommon and salt water environments in particular can give rise to corrosion: this is even noticeable at domestic level where dark spots appear caused by the mild salt solutions used during automatic dishwashing cycles.
The importance of stainless steel 1 is best reflected by the worldwide annual consumption which was reported at over 50 million tonnes in 2019. Generally recognised for its corrosion properties the material range is also widely used where strength coupled with elevated temperature resistance to oxidation.
The Principal Arc Welding Processes
There are four basic welding processes that generate the heat necessary to melt metals by striking an electric arc between an electrode and a metal. Other arc processes are in use but have limited application or lack control and quality characteristics. Initially developed towards the end of the 19th century, arc welding quickly became a commercially important processes especially in the shipbuilding sector in the second world war.
An arc is a discharge of energy between two conductors at different voltages. In welding It can be initiated by bringing the conductors, an electrode and the metals to be joined, together momentarily to create a short circuit and then drawing them apart to produce a continuous arc. Experience is necessary so that the two components do not stick together. The arc can only be maintained over a very limited separation and with manual welding this relies on operator skill. Development over the past few decades has resulted in the production of sophisticated power supplies to help stabilise the arc.
New superalloys still need careful purging during welding.
Significant developments have been made recently and have resulted in the introduction of new nickel alloys that offer major improvements in mechanical properties.
Not least is Inconel 740H (Ref 1), an alloy offering enhanced resistance to coal ash and therefore of considerable interest to fossil fuel fired boiler manufacturers.
Whilst these new materials help to expand the use of nickel-based alloys in areas where mechanical properties and corrosion resistance at elevated temperatures are mandatory, the need to maintain strict control during fusion welding remains, in order to preserve these characteristics.
The marine industry in general has been slow to embrace the 3D printing concept. The use of continuous liquid metal deposition under computer numerical control has created opportunities to produce complex shapes such as forgings and castings whilst avoiding the need for expensive tooling and the time delays in fabricating moulds.
Notwithstanding this slow start, development work at Delft Technical University in 2017 has led to the production of the world’s first metal deposited marine propeller.
The majority of published documents on 3-D printing have been restricted to high precision applications, particularly in the medical sector.
Huntingdon Fusion Techniques HFT®”s USA Partner has recently helped solve a major environmental problem in a remote area of Oregon. Construction of a new access to the Willamette River was necessary, as part of a plan to replenish a salmon hatchery1 but this necessitated removal of part of a 10-inch (254 mm) pipeline that was causing an obstruction.
The pipeline had been isolated and abandoned previously and filled with water that had probably become polluted. Simply cutting the pipeline would release over 1 million 300 thousand gallons (5,000 m3) of contaminated water into surrounding land lying within a sensitive, environmentally protected area. A decision was made to use liquid nitrogen to create ice plugs and isolate the small section of pipe causing the obstruction. The pipe could then be cut, releasing only limited contaminated water and this could be contained and removed from the site.
Pits were excavated on either side of the access exposing the pipe and the anti- corrosion coating was removed. Freezing commenced in the early morning in record high temperatures combined with little to no shade in the area.
The reactive metals by classification are zirconium, titanium and beryllium. We also include here tantalum and columbium (niobium), being from the refractory class and which also present similar challenges to the welding engineer.
Aerospace, automotive, medical and military industries are increasingly using all these materials. They have many technological attractions being durable, low density, bio-compatible and offering high corrosion resistance but they are expensive. Welding procedures need to be carefully developed and stringently applied to avoid expensive waste, rework or risk of service failure.
Successful fusion joining techniques have evolved1 since the alloys were first used in engineering applications. The majority of metallurgical problems, even considering dissimilar metal welding, have been resolved and filler materials are readily available.
There is a popular misconception that powder based additive manufacture is superior to the wire alternative. This impression has been created largely through aggressive marketing and by the technical press preferring the more glamourous powder method used for creation of body implants.
Whilst it has to be conceded that the relatively delicate powder deposition process is excellent for producing small components, often requiring no further machining, in terms of speed and cost-effectiveness the wire option wins hands-down.
Wire and Arc Additive Manufacture (WAAM), is performed by laying down progressive beads of metal under computer numerical control to create a shape. The alternative version uses a laser or electron beam as the heat source in conjunction with metal powder, Direct Metal Laser (or Electron Beam) Sintering (DMLS or DMEBS).
Since 3D printing was introduced over 30 years ago there have been a number of significant developments. Various melting techniques have been used to achieve this aim including electron beams and lasers. but one being most actively pursued currently is Wire and Arc Additive Manufacture (WAAM) using a GTAW (TIG) power source. To be specific, additive manufacturing is not the same as 3D printing!
The recent use of fusion welding as a deposition source has opened up wide ranging possibilities in manufacturing. The process is one in which metal is deposited layer-by-layer under computer control to form a three dimensional shape.
No longer is it necessary to keep an inventory of high value generic stock: parts can be customised and manufactured on demand. A recognition that components can be fabricated using WAAM technology has spawned a new industry to exploit a wide range of exciting opportunities.
Nickel is the base metal in a wide range of alloys developed primarily to provide high temperature strength and excellent corrosion resistance. Common amongst these alloys is the Hastelloy series containing chromium and molybdenum. The primary applications are in the aerospace, power generation, petrochemical, offshore and automotive sectors.
Although few problems arise with the majority of welding applications, porosity can occur and as little as 0.025% nitrogen will form pores in the solidifying weld metal. Draughts can disrupt the gas shield and atmospheric contamination will occur resulting in porosity.
Care must therefore be taken to ensure that the weld area is sufficiently protected and this is particularly relevant in site welding applications. With the gas shielded processes, gas purity and the efficiency of the gas shield must be carefully controlled.
The planned surge in new electricity power generation plant and refits across the world over the next two decades will provide outstanding opportunities for the fabrication sector. Recent innovative developments in welding equipment will support the drive towards the production of consistently better quality joints, many of which are in the safety critical class.
Over 300 nuclear reactors have been proposed of which 136 will be in China, 24 in the USA and 23 in Russia1. India’s massively delayed nuclear power programme will see a resurrection after Électricité de France (EDF), the world’s biggest electricity company, agreed build six nuclear plants in the country. The Indian Jaitapur project is expected to become the world’s biggest nuclear contract and one of the world’s largest nuclear sites. The 10,000 MW project will have six reactors of 1650 MW each.
Zirconium and its principal alloy zircaloy possess physical properties unmatched by most other metallic materials. The combination of mechanical strength, corrosion resistance and their high temperature stability make them attractive for use in sectors as diverse as biochemical, nuclear, aerospace and petrochemicals.
More specifically, zircalloy is used in the manufacture of pressure vessels and heat exchangers. The alloy has excellent resistance to most organic and inorganic acids, salt solutions, strong alkalis, and some molten salts and these properties makes it suitable for use in pumps where strength coupled with corrosion resistance is mandatory. Zirconium alloys are biocompatible, and therefore can be used for body implants: a Zr-2.5Nb alloy is used in knee and hip implants.
By far the most significant applications however are in nuclear power plant. Zirconium alloys are widely used in the manufacture of fuel rods especially in pressurised water reactors 1.
Figure 1. Zirconium alloy welded with effective inert gas protection showing no discolouration.