Author: Luke Ronne
I was a little hesitant about writing the following article. Sensitive readers may be offended; this article may contain words like steam, boiler and welding. Words like these tend to make people throw their hands up, close their eyes, and run for the hills. My personal view is boilers should instill a healthy respect but not paralytic fear. I’m also intrigued by how most people think that someone else, without a vested interest, will do a better job in welding than them. I would never let someone else weld a boiler that I’m going to sit behind and use! I have conducted inspections in boiler shops where the welders were using very expensive welding equipment to weld stainless steel piping on a setting equivalent to a standard DC inverter welder. Anyone with an interest in welding and materials prepared to do a little reading can do better! If you are interested please read on, hopefully I’ll point you in the right direction.
General model engineering boiler manufacture
With any welding the material is your starting point and that defines how you tackle the job. In model engineering boiler making there are three principal material types to choose from, namely copper, steel and stainless steel. Each one has its own set of design challenges, and without going into too much technical detail they can be summarized as follows:
Copper: copper is very expensive and classically all joints are made using silver solder which will easily double the price of the boiler. I have personally made a copper boiler by TIG welding the main joints and silver soldering the tubes and stays into place which cut the costs drastically. This works well for smaller boilers where material cost won’t break the bank. Galvanic corrosion  might be an issue if incorrect materials are used for the fittings; in general this is a problem with all boilers. My first boiler was copper and it is still running.
Steel: Steel is generally the cheaper option but corrosion allowance is necessary in the design making the sidewalls thick. Rust tends to seal the tubes if they expanded in place, but you do need to be careful when storing the loco for long periods to prevent continued corrosion. This wasn’t an option for the Sterling due to sizing issues. Because the main wheels extend past the centre line of the boiler the wrapper is constrained. Using steel I would have had to remove a number of heating tubes to compensate for the corrosion allowance, drastically decreasing the thermal efficiency of the boiler.
Stainless steel: Stainless steel has stress corrosion issues where areas that have been sensitised could fail in the presence of chlorides. This sensitization is typically due to poor welding and/or when the incorrect stainless is used with the incorrect filler rods. I chose to use stainless for the Sterling based on the cost and size constraints.
For interest the original boiler during Sterling’s time had a copper firebox, brass tubes and the wrapper was made from a type of wrought iron. The only welding they had back then was forge welding with the stays and other joints generally riveted or brazed. These boilers were run (we think) at around 140Psi (9.6Bar). Knowing what we know now, these boilers seemed very dangerous but they had a typical life span of 12 years running on the express line 7 days a week! Most of the failures were on the crown  The crown is the top section of the firebox wrapper. which immediately dumped steam onto the fire putting it out. A typical Stirling boiler is shown below.
The Stirling boiler
Before I get to the manufacturing bit, which is what you’re probably interested in, I need to highlight some points with my boiler. My boiler was redesigned using the Stainless Steel Australian Model Engineering boiler code, bringing it in line with the code and updating some of the out-dated manufacturing techniques. The design was presented to the boiler inspector along with the material certificates, welding filler rods, welding procedures, pictures of all the weld preps and fitting types along with the calculations for the fittings and the safety valve. Test welds of all types of joints were made using the same material and filler rods, bent 90Deg on the weld and presented to the boiler inspector. The welding procedures were drawn up by a very close friend of mine, who is a physical metallurgist and a welding guru who has been responsible for some very scary boiler and high pressure vessel applications in heavy industry. Once all of this is done only then can you start on welding the actual boiler.
As the boiler was assembled photos were taken to document the process and these were added to the data pack for the boiler which was filed with the boiler inspector. I am privileged enough to have one of the high frequency TIG welding sets which gives incredibly good control over the welding process. The welds were done using DC pulse welding with a moderate pulse frequency. This narrows your welding arc, allowing you to weld at the required amperage but decreases your overall heat input decreasing the heat affected zone and decreasing the chances of sensitization. The welding distortion is also decreased due to the lower overall heat input. Unfortunately you can’t use the foot pedal because that would nullify the welding procedures, but the downslope and upslope setting can be used which decreased the damage (sensitization) caused by stop start welding. The pre and post gas flow was also used to remove air from the arc area before starting the weld which would again cause problems. There’re a lot of settings and knobs on these welders, far too many to mention in an article such as this, but the manual that comes with the welder combined with reading a few welding books will get most people started. The point that needs to be made is simply with critical welds you need to be critical about your welding!
When welding TIG do yourself a favour and buy a good quality auto darkening helmet. You’ll also need TIG welding gloves for feeding the filler rod, filler rods, welding rods (red tip for stainless and mild steel) etc. Most welding consumables are obtainable from Adendorff.
Finally after all that the boiler plugs are fitted and the boiler is hydraulically tested to twice working pressure, witnessed by the boiler inspector for no less than one hour. When I tested my boiler the pump was removed and the boiler was left in the sun and the pressure actually increased due to the expansion of the water, that’s a thumbs up for no leaks.
The other major welded item was the tender tank, and here I could experiment a little more with my welding. All the welds were fusion welds with a very high frequency and low base amps to minimize any distortion. The plates are relatively thin which makes thermal distortion the major concern, but with my fancy welder I had almost no noticeable plate movement.
Mild steel TIG welding
You can of course weld mild steel or even dissimilar materials with these welders with the normal DC electrode negative setup. The trick with mild steel is to get rid of the scale, which is essentially oxides, either mechanically or by using an acid. Just make sure the weld area is clean for TIG welding. Failing this you will typically cause porosity in the welds.
Heat treating metals
Having a basic understanding of materials obtained from casting and welding allows the model engineer to change materials to suit a specific purpose. A few years ago I purchased some taps from England and I was very disappointed with the quality. With my comments falling on deaf ears, I decided to make my own taps and have never looked back. Most of my taps are made from mild steel just because I buy lengths and generally I have the size I’m looking for. Any engineer reading this would say that there’s not enough carbon in mild steel to harden it, and they would be right. I add carbon by placing the tap in a sealed metal container fashioned from some pipe surrounded by carbon (my actual recipe is 40% graphite, 59% ground anthracite and 1% soda ash). I place the pipe in one of those Weber fire starter tube and I let it cook. When the fire is ready the coals are placed in the braai, the contents of the pipe are thrown into water and of course we have a nice braai with the remaining coals.
The taps are then cleaned and tempered in a conventional oven at around 200°C, typically slipped next to a chicken when the missus is not looking. Some of these taps have tapped a few holes through stainless and they’re still going strong.
The next article is one I’m really looking forward to. It’ll cover the painting and the detailing of the loco. It’ll be more pictures than my babbling, with the model pictures beside pictures of the real loco for comparison.Galvanic corrosion: Is an electrochemical process where one material corrodes preferentially when there is a potential electrical loop in the system. A good example is when Zinc is leached from brass in a boiler causing failure of the brass fittings.  The crown is the top section of the firebox wrapper.