Views: 6 Author: DURMAPRESS Publish Time: 2022-08-22 Origin: DURMAPRESS
How to Choose Welding Materials for Stainless Steel?
Table of Contents
The effect of four types of stainless steel and alloy elements
How to choose the riant stainless steeweding materias?
If you want to match the welding material with the base materialhow to choose welding material to weld 304 stainless steel?
How to weld stainless steel and carbon steel? What is a suitable pre-weld cleaning operation?
What is the correct post-weld cleaning operation? Why do stainless steel weldments rust? Why is stainless steel welding wire magnetic? How to weld duplex stainless steel more easily?
How to control carbide precipitation in austenitic stainless steel? How to prepare for the selection of welding materials?
The effect of four types of stainless steel and alloy elements
There are four types of stainless steel:austeniticmartensiticferriticand duplex stainless steel (Table 1).
This is based on the metallographic structure of stainless steel at room temperature.
When the mild steel is heated to 1550°Fits structure changes from a ferrite phase to an austenite phase at room temperature.
When cooledthe structure of the low carbon steel is transformed into ferrite again.
The austenite structure existing at high temperature is non-magnetic and its strength is
lower and its toughness is better than roomtemperature ferrite structure.
When the Cr content in the steel is areater than 16%the ferrite structure at room
temperature is fixed so that the steel maintains the ferrite state in all temperature ranges Therefore. it is called ferritic stainless steel
When the Cr content is more than 17% and the Ni content is more than 7%the austenite phase is fixed,so that the austenite state is maintained from the low temperature to the almost melting point.
Austenitic stainless steel is usually called“Cr-Nitypeand martensitic and ferritic stainless steels are directly called“Cr"type
Elements in stainless steel and filler metals can be divided into austenite-forming elements and ferrite-forming elements.
The most important austenite-forming elements areNiCMand Nand the ferrite
forming elements are Cr SiMo,and Nb.The element content can be adjusted to control the ferrite content in the weld.
Austenitic stainless steel is easier to weld and has better welding quality than stainless steel containing less than 5% Ni
The welded joints of austenitic stainless steel have good strength and toughness and generally do not require pre-heating and post-weld heat treatment.
In the field of stainless steel welding,austenitic stainless steel accounts for 80% of the total amount of stainless steelso the focus of this article is the welding of austenitic stainless steel.
How to choose the right stainless steel welding materials?
If the base material is the same,the first criterion ismatching the base material”.
For example,when welding 310 or 316 stainless steel you need to select the corresponding welding material.
When welding dissimilar materials,follow the guidelines for selecting a base metal with a high content of alloying elements.Such as welding 304 and 316 stainless steel, you have to choose 316 type welding consumables.
However, there are many special cases that do not follow the principle ofmatching the base metal". In this caseremember to“checkthe welding material selection table”.
For example, type 304 stainless steel is the most common base metal but there is no 304 electrode.
If you want to match the welding material with the base material, how to choose welding material to weld 304 stainless steel?
When welding 304 stainless steel,use type 308 welding materialbecause the additional elements in 308 stainless steel can better stabilize the weld area.
308L is also an acceptable choice.L means low carbon content3XXL stainless steel means carbon content ≤0.03%while standard 3XX stainless steel can contain up to
0.08% carbon content
Since L-type welding materials belong to the same type asnon-L-type welding materials
manufacturers should especially consider usinaL-type weldina consumables because
their low carbon content can reduce the tendency for intergranular corrosion(Figure 1)
In fact, I believe that if manufacturers want to upgrade their products,L-shaped welding materials will be more widely used.
Manufacturers using GMAW welding method can also consider using 3XXSi type welding materials because Si can improve wettability(Figure2).
In the case where the weldment has a high bulge or the weld pool is poorly connected at the toe of the fillet or lap weldthe use ofSi-containing gas shielded welding wire can wet the weld and increase the deposition rate.
Figure 2 In order to improve the wettability of the welding material in GMAW weldinga Si containing welding wire is usedsuch as 308L Si or 316L Si
If you consider carbide precipitation,you can choose a 347 type welding materia containing a small amount of Nb element (the last question).
How to weld stainless steel and carbon steel?
In order to reduce costs,some structural parts will be welded with a corrosion-resistant layer on the surface of carbon steel.
When welding base alloys containing no alloying elements and base alloys containing alloying elements,a welding alloy with a higher alloy content is used to balance the dilution rate in the weld.
When welding carbon steel and 304 or 316 stainless steelas well as other dissimilar stainless steelsTable 2,309L welding consumables are considered in most cases.lf higher Cr content is desiredthen type 312 is used.
Table 2 Stainless steels with high alloy content of 309L and 312 are suitable for welding
stainless steel and carbon stee
It should be noted that the thermal expansion rate of austenitic stainless steel is 50% higher than that of carbon steel.
When welding,the difference in thermal expansion rate will cause internal stress which will cause cracks.
At this time, you need to choose the appropriate welding material or specify the appropriate welding process(Figure 3).
Figure 3 When welding carbon steel and stainless steel,warpage deformation due to different thermal expansion rates needs greater compensation
What is a suitable pre-weld cleaning operation?
When welding to other materials,first use a chloride-free solvent to remove oilmarks, and dust. In addition, the first thing to pay attention to when welding stainless steel is to avoid contamination by carbon steel and affect the corrosion resistance.Some companies store
stainless steel and carbon steel separately to avoid cross-contamination.
When cleaning the area around the grooveuse a special sanding wheel and brush for
stainless steel. Sometimes secondary cleaning of the joint is required.Because electrode compensation is more difficult when welding stainless steel than when welding carbon steel,thereforejoint cleaning is veryimportant
What is the correct post-weld cleaning
operation? Why do stainless steel weldments rust?
First of all, let’s recall that the reason why stainless steel does not rust:The reaction of Cr and O generates a dense oxide layer on the surface of the materialwhich plays a protective role.
Stainless steel is rusted because of the precipitation of carbides (see the last question) and heating during the welding processresulting in the formation of iron oxides on the surface of the weldment.
In the welded state, a perfect weldment may also cause undercuts in rusty areas at the boundaries of the welding heat affected zone within 24 hours.
Therefore, in order to regenerate new chromium oxidesstainless steel needs to be
polishedpickledsanded or scrubbed after welding
It is important to emphasize that the sander and brush must be proprietary.