Gas Composition OR Confusion?!

   When a concept is not clearly understood or misunderstood or half boiled!!!, there we get easily confused. There are some people really to confuse you by triggering a little twisted question and make you to get confused. To avoid loosing the conversation we start argue to prove our self correct. So it is important and vital to understand the basic fundamentals.

   What is Composition?

   Our earth is composed of many gases like Nitrogen,Oxygen, Co2, Hydrogen etc at certain percentages. So Composition of Atmospheric air is 78% Nitrogen + 21% Oxygen + 1% Argon + Other gases. Or we can say the Atmospheric air is mixture of these gases.

   Let us take another Golden Example!!!

   We know Gold and is available in different quality levels such as 24kt, 22kt, 18kt etc. 24kt gold is pure gold and nothing mixed to it and stands Single. In 22kt and 18kt, Gold is mixed with copper at certain amounts to make the gold suitable for ornaments. So 22kt or 18kt gold is Mixed with copper or Composed of copper at certain percentages ( 22 parts gold / 2 parts copper or 18 parts gold / 6 parts copper). 

If say, I have 24kt Gold means it is pure gold. Single

If say, I have 22kt Gold means it is a mixture or composition of Gold and Copper ( 91.66 % Gold + 8.33% Copper). 91.66% of Gold is Pure Gold and balance 8.33% is Pure Copper.

If say, I have 18kt Gold means it is a mixture or composition of Gold and Copper (75 % Gold + 25% Copper). 75% of Gold is Pure Gold and balance 25% is Pure Copper.

STOP! STOP!!. How is 18kt gold composition arrived ?

18 divided by 24 and multiplied by 100 gives in percentage

   Welding Gas Composition

   Similarly in Gas Welding process(es), the Gas Composition is an essential variable. WPS shall always be written as either Single or Mixture.

if Gas mixtures or Compositions used, shall always be specified such as 98% Argon + 2 % H2 etc. Not the Purity of the Argon Gas ie 99.99%.

If only argon is used, specify in the WPS as single gas ( some times Client may ask you specify the Purity levels for SS, DSS, SDSS or Titanium metals)

   Composition Vs Purity 

   Composition of Gas will affect the Arc Characteristics, thereby influence arc energy, penetration, weld profile, weld fluidity etc

Purity of Gas will affect the weld quality and corrosion properties

I tried to simplify the topic as much as possible and hope the lesson can be understood easily and clearly

Please leave your comments in the box below

Preheating Temperature Requirements

  We have seen the purpose for Preheating and why we should not do preheating by gas cutting torch in our earlier Lessons.

Click Here for purpose of Preheating

Click Here for Preheating by Gas Cutting Torch

   In this lesson we are going to learn the requirements for Preheating. The preheating requirements predominantly comes from the following:-

- Material Composition

- Material Thickness

- Carbon Equivalent

- Weather conditions

Material Composition

   When the material composition consists of Carbon and other alloying elements such as Chromium, Molybdenum, Vanadium may influence the weldability of steels depends upon their composition. These steels will require preheating irrespective of thickness. (Low alloy steels-LAS)

Material Thickness

   Material thickness also will influence the requirement for preheating since to reduce the thermal gradient in the weld joint. when the material thickness greater than 20 mm  then preheating may be necessary to bring down the thermal gradient.

Carbon Equivalent (CE)

   This is the most influential factor in deciding the preheating requirements for Carbon steel Materials. The higher the carbon equivalent, will have less weldability factor and thus require preheating. Generally CE of more than 0.4 may require preheating.
Carbon Equivalent is calculated based upon Carbon, Manganese, Chromium, Molybdenum, Nickel, Vanadium etc.

The formula for calculating CE is

Weather Conditions

   Weather conditions are very important to decide whether preheating to be applied or not. If moisture or water or ice present on the weld surface to be joined, then it must be removed before start welding. Or otherwise, if the humidity is high i.e above 90% preheating may be necessary in such situations.

Remember: Preheating is MINIMUM temperature to be applied on the weld joint.

Minimum the Preheating Temperature

Generally the following specifies the minimum preheating temperature:-

- Construction Codes and Standards

- Client Specifications

- WPS

- Weather Conditions

Quiz Question: 

A client Specification requires preheating to 100 deg C. The construction Code requirements for Preheating is 75 deg C. WPS Qualified with 80 Deg C. What Preheat temperature you will apply to the weld joint?

Answer Options:

a) Apply Preheat 75 deg C since it is the governing Code for Construction
b) Apply preheat 100 deg C, since it is client requirement otherwise Client will raise NCR
c) Apply 80 deg C, since the WPS is Qualified with 80 deg C only.
d) Revise WPS preheat temperature to 100 deg C and preheat to 100 deg C

These kind of situations will arise every often to all the Welding inspectors and the right decision is important. Get involved in the Quiz

Post Your replies or queries in the comments box below

Answer for Preheating by Gas Cutting Torch

Here is the Answer for the previous Question 

Preheating by Gas cutting Torch (Oxy fuel Cutting Torch) is prohibited due to that

- Excess oxygen in the flame will carburise the material surface and   thereby reducing the material strength
- The surface hardness will be reduced
- May locally overheat and make depression on the material surface by removing the surface material due to gas pressure
- Control of preheat temperature is difficult
- The material surface may become oxidised and may produce weld defects ( Non-Metallic Inclusions such as Oxide Inclusions)

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Preheating by Gas Cutting Torch

Preheating of Steels in Welding is being carried out sometimes. This preheating is done to achieve

- To aid the fusion of welding
- To remove the moisture
- To slowdown the cooling rate
- To reduce thermal gradient difference
- To reduce the HAZ hardness
- To reduce the HAZ width
- To minimise the risk of Hydrogen crack
- To reduce the distortion

Preheating can be applied in many ways such as Preheating Torch burners, Induction heating, Thermocouple Heating, etc

However the Preheating SHALL NOT be done with Gas Cutting Torch. WHY???!!!

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